Problem with ``tikzpicture`` in beamer in creating a matrix

I’m trying to use tikz in beamer as follows but get an error "Undefined control sequence. end{frame}"

My MWE is:

documentclass{beamer}
setbeamercovered{highly dynamic}
%setbeamertemplate{bibliography item}{}

usepackage{lipsum, ragged2e, etoolbox}
usepackage{lmodern}
usepackage{graphicx}      % include this line if your document contains figures
%usepackage{natbib}        % required for bibliography
usepackage{booktabs} % For formal tables
usepackage{dsfont, color, soul, tikz, amsmath, amssymb, amsfonts, amsthm, graphicx, arydshln, fixmath, mathtools, multirow, boldline, xcolor, colortbl, url, algorithm, algpseudocode, subfig, media9, hyperref, blindtext, import, tcolorbox}
usepackage{caption}
captionsetup[figure]{labelformat=empty}
usetikzlibrary{arrows, fit, matrix, positioning, shapes, backgrounds}

addtobeamertemplate{footnote}{}{vspace{2ex}}
makeatletter
defhrulefill{leavevmodeleadershrule height 2pthfillkernz@}
makeatother

newcommandblfootnote[1]{%
    begingroup
    renewcommandthefootnote{}footnote{#1}%
    addtocounter{footnote}{-1}%
    endgroup
}
    
newcolumntype{L}[1]{>{raggedrightarraybackslash}p{#1}}
newcolumntype{C}[1]{>{centeringarraybackslash}m{#1}}
newcolumntype{R}[1]{>{raggedleftarraybackslash}p{#1}}

usetheme{Madrid}

setbeamercolor{section in head/foot}{parent=palette quaternary}

makeatletter
setbeamertemplate{footline}
{%
    vskip-9ex%
%   begin{beamercolorbox}{}
%       hfillusebeamercolor[fg]{navigation symbols dimmed}%
%       insertslidenavigationsymbol
%       insertframenavigationsymbol
%       insertsubsectionnavigationsymbol
%       insertsectionnavigationsymbol
%       insertdocnavigationsymbol
%       insertbackfindforwardnavigationsymbol
%   end{beamercolorbox}%    
%   begin{beamercolorbox}[ht=3ex,dp=4ex]{section in head/foot}%
%       insertnavigation{.9paperwidth}
%   end{beamercolorbox}%
leavevmode%
hbox{%
    begin{beamercolorbox}[wd=0.9paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
        insertnavigation{.9paperwidth}
%       usebeamerfont{author in head/foot}{}
    end{beamercolorbox}%
    begin{beamercolorbox}[wd=0paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
        usebeamerfont{title in head/foot}{}
    end{beamercolorbox}%
    begin{beamercolorbox}[wd=.1paperwidth,ht=2.25ex,dp=1ex,center]{date in head/foot}%
        insertframenumber{} / inserttotalframenumberhspace*{1ex}
end{beamercolorbox}}%
vskip0pt%
}%
makeatother

%AtBeginSection[]{
%   begin{frame}
%   vfill
%   centering
%   begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
%       usebeamerfont{title}insertsectionheadpar%
%   end{beamercolorbox}
%   vfill
%end{frame}
%}

%makeatother
%setbeamertemplate{footline}
%{
%   leavevmode%
%   hbox{%
%       begin{beamercolorbox}[wd=0.9paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
%           usebeamerfont{author in head/foot}{}
%       end{beamercolorbox}%
%       begin{beamercolorbox}[wd=0paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
%           usebeamerfont{title in head/foot}{}
%       end{beamercolorbox}%
%       begin{beamercolorbox}[wd=.1paperwidth,ht=2.25ex,dp=1ex,center]{date in head/foot}%
%           insertframenumber{} / inserttotalframenumberhspace*{1ex}
%   end{beamercolorbox}}%
%   vskip0pt%
%}
%makeatletter

title{My title}

% A subtitle is optional and this may be deleted
%subtitle{Optional Subtitle}

author{Me}
% - Give the names in the same order as the appear in the paper.
% - Use the inst{?} command only if the authors have different
%   affiliation.

institute[University] % (optional, but mostly needed)
{
  %inst{1}%
  Electrical and Computer Engineering Department
  vspace{.3cm}
  }


date{October 1, 2019}

begin{document}

begin{frame}
  begin{equation}
lambda_{max}(W_c) = begin{bmatrix}
begin{tikzpicture}
matrix (m) [
matrix of math nodes,
nodes in empty cells,
minimum width=width("998888"),
] {
    1.5678 & 9.1645 & 5.2552 & 3.6413
    4.3001 & 1.5605 & 5.2552 & 3.6413
    6.0162 & 4.0937 & 1.5561 & 3.6413
    10.0278 & 5.6221 & 3.8836 & 1.5504
};
draw (m-2-1.south west) rectangle (m-2-4.north east);
draw (m-1-3.north west) rectangle (m-4-3.south east);
scoped[on background layer]
{
    node[fill=gray!30, fit=(m-2-3)(m-2-3) ]   {};
}
end{tikzpicture}
end{bmatrix}
end{equation}
end{frame}

end{document} 

documentclass{beamer}
setbeamercovered{highly dynamic}
%setbeamertemplate{bibliography item}{}

usepackage{lipsum, ragged2e, etoolbox}
usepackage{lmodern}
usepackage{graphicx}      % include this line if your document contains figures
%usepackage{natbib}        % required for bibliography
usepackage{booktabs} % For formal tables
usepackage{dsfont, color, soul, tikz, amsmath, amssymb, amsfonts, amsthm, graphicx, arydshln, fixmath, mathtools, multirow, boldline, xcolor, colortbl, url, algorithm, algpseudocode, subfig, media9, hyperref, blindtext, import, tcolorbox}
usepackage{caption}
captionsetup[figure]{labelformat=empty}
usetikzlibrary{arrows, fit, matrix, positioning, shapes, backgrounds}

addtobeamertemplate{footnote}{}{vspace{2ex}}
makeatletter
defhrulefill{leavevmodeleadershrule height 2pthfillkernz@}
makeatother

newcommandblfootnote[1]{%
    begingroup
    renewcommandthefootnote{}footnote{#1}%
    addtocounter{footnote}{-1}%
    endgroup
}
    
newcolumntype{L}[1]{>{raggedrightarraybackslash}p{#1}}
newcolumntype{C}[1]{>{centeringarraybackslash}m{#1}}
newcolumntype{R}[1]{>{raggedleftarraybackslash}p{#1}}

usetheme{Madrid}

setbeamercolor{section in head/foot}{parent=palette quaternary}

makeatletter
setbeamertemplate{footline}
{%
    vskip-9ex%
%   begin{beamercolorbox}{}
%       hfillusebeamercolor[fg]{navigation symbols dimmed}%
%       insertslidenavigationsymbol
%       insertframenavigationsymbol
%       insertsubsectionnavigationsymbol
%       insertsectionnavigationsymbol
%       insertdocnavigationsymbol
%       insertbackfindforwardnavigationsymbol
%   end{beamercolorbox}%    
%   begin{beamercolorbox}[ht=3ex,dp=4ex]{section in head/foot}%
%       insertnavigation{.9paperwidth}
%   end{beamercolorbox}%
leavevmode%
hbox{%
    begin{beamercolorbox}[wd=0.9paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
        insertnavigation{.9paperwidth}
%       usebeamerfont{author in head/foot}{}
    end{beamercolorbox}%
    begin{beamercolorbox}[wd=0paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
        usebeamerfont{title in head/foot}{}
    end{beamercolorbox}%
    begin{beamercolorbox}[wd=.1paperwidth,ht=2.25ex,dp=1ex,center]{date in head/foot}%
        insertframenumber{} / inserttotalframenumberhspace*{1ex}
end{beamercolorbox}}%
vskip0pt%
}%
makeatother

%AtBeginSection[]{
%   begin{frame}
%   vfill
%   centering
%   begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
%       usebeamerfont{title}insertsectionheadpar%
%   end{beamercolorbox}
%   vfill
%end{frame}
%}

%makeatother
%setbeamertemplate{footline}
%{
%   leavevmode%
%   hbox{%
%       begin{beamercolorbox}[wd=0.9paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
%           usebeamerfont{author in head/foot}{}
%       end{beamercolorbox}%
%       begin{beamercolorbox}[wd=0paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
%           usebeamerfont{title in head/foot}{}
%       end{beamercolorbox}%
%       begin{beamercolorbox}[wd=.1paperwidth,ht=2.25ex,dp=1ex,center]{date in head/foot}%
%           insertframenumber{} / inserttotalframenumberhspace*{1ex}
%   end{beamercolorbox}}%
%   vskip0pt%
%}
%makeatletter

title{My title}

% A subtitle is optional and this may be deleted
%subtitle{Optional Subtitle}

author{Me}
% - Give the names in the same order as the appear in the paper.
% - Use the inst{?} command only if the authors have different
%   affiliation.

institute[University] % (optional, but mostly needed)
{
  %inst{1}%
  Electrical and Computer Engineering Department
  vspace{.3cm}
  }


date{October 1, 2019}

begin{document}

begin{frame}[fragile] %<--- Add fragile here
  begin{equation}
lambda_{max}(W_c) = begin{bmatrix}
begin{tikzpicture}
matrix (m) [
matrix of math nodes,
nodes in empty cells,
minimum width=width("998888"),
] {
    1.5678 & 9.1645 & 5.2552 & 3.6413
    4.3001 & 1.5605 & 5.2552 & 3.6413
    6.0162 & 4.0937 & 1.5561 & 3.6413
    10.0278 & 5.6221 & 3.8836 & 1.5504
};
draw (m-2-1.south west) rectangle (m-2-4.north east);
draw (m-1-3.north west) rectangle (m-4-3.south east);
scoped[on background layer]
{
    node[fill=gray!30, fit=(m-2-3)(m-2-3) ]   {};
}
end{tikzpicture}
end{bmatrix}
end{equation}
end{frame}

end{document}