Btukfyl

 documentclass{interact}

 usepackage{epstopdf}% To incorporate .eps illustrations using PDFLaTeX, etc.

 usepackage{subfig}% Support for small, `sub' figures and tables

 %usepackage[nolists,tablesfirst]{endfloat}% To `separate' figures and tables 

 %from text if required

 %usepackage[doublespacing]{setspace}% To produce a `double spaced' document if 

 %required

 %setlengthparindent{24pt}% To increase paragraph indentation when line spacing 

 %is doubled



 usepackage[longnamesfirst,sort,comma,authoryear]{natbib}% Citation support 

 %using natbib.sty

 bibpunct[, ]{(}{)}{;}{a}{,}{,}% Citation support using natbib.sty

 renewcommandbibfont{fontsize{10}{12}selectfont}% To set the list of 

 %references in 10 point font using natbib.sty



 %usepackage[natbibapa,nodoi]{apacite}% Citation support using apacite.sty. 

 %Commands using natbib.sty MUST be deactivated first!

 %setlengthbibhang{12pt}% To set the indentation in the list of references 

 %using apacite.sty. Commands using natbib.sty MUST be deactivated first!

 %renewcommandbibliographytypesize{fontsize{10}{12}selectfont}% To set the 

 %list of references in 10 point font using apacite.sty. Commands using 

 %natbib.sty 

 %MUST be deactivated first!



 theoremstyle{plain}% Theorem-like structures provided by amsthm.sty

 newtheorem{theorem}{Theorem}[section]

newtheorem{lemma}[theorem]{Lemma}

newtheorem{corollary}[theorem]{Corollary}

newtheorem{proposition}[theorem]{Proposition}



theoremstyle{definition}

newtheorem{definition}[theorem]{Definition}

newtheorem{example}[theorem]{Example}



theoremstyle{remark}

newtheorem{remark}{Remark}

newtheorem{notation}{Notation}



usepackage{amsmath}

usepackage{hyperref}

usepackage{makecell}

usepackage{graphicx}

usepackage{longtable}

usepackage{supertabular}

usepackage{morefloats}

usepackage{tablefootnote}

usepackage[final]{pdfpages}

usepackage{import}

usepackage{filecontents}

usepackage{multirow}

usepackage{booktabs}

usepackage{siunitx}

usepackage{soul}

usepackage{textcomp}

usepackage{threeparttable}

usepackage[natbibapa,nodoi]{apacite}

setlengthbibhang{12pt}

renewcommandbibliographytypesize{fontsize{10}{12}selectfont}

%usepackage{cite}

renewcommand{bibname}{References}





begin{document}

bibliographystyle{apacite}

bibliography{thesis}



end{document}

Just made these changes and doesn't give any more errors:

First got rid your documentclass (replaced by article, but not asking to do it... just could not -didn't want- to search for it). I had to get rid (comment out) the `teoremstyle` too ... but if it is defined in there you could use it.

Second got rid of the usepackage{epstopdf} ... If you really need it, add it, but add graphicx or something before it.

Third loaded apacite before natbib to get rid some remaining errors.

Fourth according to this post: https://tex.stackexchange.com/a/263800/120578
I had to remove the options from natbib package while loading.

The following code (with the above changes) doesn't give errors.

documentclass{article}

%usepackage{epstopdf}% To incorporate .eps illustrations using PDFLaTeX, etc.

usepackage{subfig}% Support for small, `sub' figures and tables

%usepackage[nolists,tablesfirst]{endfloat}% To `separate' figures and tables 

%from text if required

%usepackage[doublespacing]{setspace}% To produce a `double spaced' document if 

%required

%setlengthparindent{24pt}% To increase paragraph indentation when line spacing 

%is doubled

usepackage[natbibapa,nodoi]{apacite}

usepackage{natbib}% Citation support 



%using natbib.sty

bibpunct[, ]{(}{)}{;}{a}{,}{,}% Citation support using natbib.sty

renewcommandbibfont{fontsize{10}{12}selectfont}% To set the list of 

%references in 10 point font using natbib.sty



%usepackage[natbibapa,nodoi]{apacite}% Citation support using apacite.sty. 

%Commands using natbib.sty MUST be deactivated first!

%setlengthbibhang{12pt}% To set the indentation in the list of references 

%using apacite.sty. Commands using natbib.sty MUST be deactivated first!

%renewcommandbibliographytypesize{fontsize{10}{12}selectfont}% To set the 

%list of references in 10 point font using apacite.sty. Commands using 

%natbib.sty 

%MUST be deactivated first!



%theoremstyle{plain}% Theorem-like structures provided by amsthm.sty

newtheorem{theorem}{Theorem}[section]

newtheorem{lemma}[theorem]{Lemma}

newtheorem{corollary}[theorem]{Corollary}

newtheorem{proposition}[theorem]{Proposition}



%theoremstyle{definition}

newtheorem{definition}[theorem]{Definition}

newtheorem{example}[theorem]{Example}



%theoremstyle{remark}

newtheorem{remark}{Remark}

newtheorem{notation}{Notation}



usepackage{amsmath}

usepackage{hyperref}

usepackage{makecell}

usepackage{graphicx}

usepackage{longtable}

usepackage{supertabular}

usepackage{morefloats}

usepackage{tablefootnote}

usepackage[final]{pdfpages}

usepackage{import}

usepackage{filecontents}

usepackage{multirow}

usepackage{booktabs}

usepackage{siunitx}

usepackage{soul}

usepackage{textcomp}

usepackage{threeparttable}



setlengthbibhang{12pt}

renewcommandbibliographytypesize{fontsize{10}{12}selectfont}

%usepackage{cite}

renewcommand{bibname}{References}





begin{document}

TEST

bibliographystyle{apacite}

bibliography{thesis}



end{document}