Latex Error: Command `mathdollar' already defined

You really need to load MnSymbol and wasysym? I assume you are using (Lua/Xe)LaTeX and have better ways to access those symbols (if you need them).

documentclass[,landscape]{tikzposter}
usepackage{empheq,mathtools}
usetheme{Basic} % b->B
author{Marya Blabla}
title{Huge Training on Typesetting Math in Ms Word}
institute{Blabla University}
usepackage{fontspec}
newfontfamilyDejaSans{DejaVu Sans}

%usepackage{MnSymbol,wasysym}
begin{document}
maketitle

block{LARGE Wait Math in Ms Word?}{
LARGE
begin{itemize}
item Really?
item You must be joking!!!!!Whoaaaa
item Sign me up!
end{itemize}
}

note[rotate=8, connection, width = 7cm,
]{LARGE Only in 2Hours! }

begin{columns}

column{0.7}
block{LARGE But, what will be covered?}{LARGE
begin{itemize}
item Almost everything
item Everything?
item Yes, Almost Everything!
begin{empheq}[left=empheqlbrace]{align}
  begin{rcases}
displaystyle f(x)= frac{a_0}{2}+sum_{n=1}^{infty}[a_n cos(nx) + b_n sin(nx)] 
displaystyle a_0 =frac{1}{pi}int_{-pi}^{pi}f(x) dx
  end{rcases}
  
  begin{rcases}
displaystyle f(x) = sum_{n=-infty}^{infty} c_n e^{inx}
displaystyle c_n  = frac{1}{2pi} int_{-pi}^{pi} f(x) e^{-inx}dx,quad n=0,pm1,pm2,ldots
  end{rcases}
end{empheq}
end{itemize}
}

column{0.3}
block{LARGE Ok, who is the Trainer}{LARGE
begin{itemize}
item Your trainer see below {DejaSans ?}

item Really???
end{itemize}
}

end{columns}

end{document}