The maketitle command fails when using the APA7 package

• don't use scrlayer-scrpage with the apa7 class. The class already uses the fancyhdr package for the headers and footers, so you can simply use the macros from this package

• don't use a floating figure environment inside the title macro, that makes no sense

• load hyperref after the other packages

• are you really sure that your encoding is utf8x? That's highly usual in this millennium...

• don't abuse for line breaks

• apa7 has it's own mechanism to deal with multiple authors and add information like the curse or lecturer to the title page

--

documentclass[a4paper,12pt,stu,noextraspace]{apa7}
usepackage[english]{babel}
usepackage{amsmath}
allowdisplaybreaks[4]
% usepackage{inputenc}
usepackage[utf8x]{inputenc}
usepackage{csquotes}

% package for including graphics with figure-environment
usepackage{graphicx}
setlength{intextsep}{5pt}
usepackage{float} 
usepackage{placeins}
usepackage{subfigure} 
usepackage[authoryear,round]{natbib}
usepackage{setspace}
usepackage{bookmark}
linespread{1.25}

% colors for hyperlinks
% colored borders (false) colored text (true)
usepackage{hyperref}
hypersetup{colorlinks=true,citecolor=black,filecolor=black,linkcolor=black,urlcolor=black}

setcounter{secnumdepth}{5}


lhead{Name1,Name2,Name3}%
rhead{today}%
cfoot{thepage}%



begin{document}
    title{Project II}
    
    % if you are the only author, you might use the following
    % author{Name of student}  
    
    % Insert here your name and correct mail address
    authorsnames{href{mailto:a}{1},href{mailto:a}{2},href{mailto:a}{3}}
    authorsaffiliations{}
%    
%    

%    % name of the course and module
    duedate{today}
        course{Course: 0}
        professor{Lecturer: 0}

% begin{document}
{flushleft  includegraphics[width=0.26textwidth]{example-image-duck}}

maketitle 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% CONTENT PAGE
newpage
tableofcontents
    

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%          Problem Set 2
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
newpage

pdfbookmark{contentsname}{Problem Set 2}
section*{Problem Set 2}
label{sec:ps2}

section{Problem}
label{sub:Problem}

.......
end{document}