Attention: I'm sorry to be such a novice with LaTex.
I am using my Uni's template for my research paper. I am having to use micrometers a lot, and reading through the forum posts here it seems like I should be able to just use \usepackage{siunitx} then say \SI{1.55}{\micro\metre} (taken directly from the best answer of another post) in the document. However if I add this to the start of the document it all goes to hell (short list of errors given in image). The only difference from working to this is the use package, and it happens in both compilers I have. I have included the code down to line 34 where it seems to take issue with the main document.
\documentclass[12pt,a4paper]{uefrep}
%
\usepackage[english]{babel}
\usepackage{siunitx}
\usepackage[T1]{fontenc}
\usepackage{cite,url}
\usepackage[dvips]{graphicx}
\usepackage[tbtags]{amsmath}
\usepackage{psfrag}
\usepackage{color}
\usepackage{ae,aecompl}
\usepackage{bookmark}
%\usepackage[a-1b]{pdfx} %uncomment for thesis
\usepackage{hyperref}
\hypersetup{colorlinks=true,linkcolor=blue,citecolor=red,urlcolor=cyan,breaklinks=true}
%\usepackage{pdfpages} %optional for inserting multipage pdf files to document
%
\pagestyle{plain}
\bibliographystyle{uefunsrt}
\setlength{\textheight}{20.7cm}
\setlength{\textwidth}{15.0cm}
\setlength{\hoffset}{-2mm} % Because the location of left margin depends on the printer, adjust this so that the left margin is 32 mm.
%\setlenght{\voffset}{2mm} % If necessary, adjust the top margin to 45 mm with this parameter.
\renewcommand{\baselinestretch}{1.2}
%
\begin{document}
%
\pagenumbering{roman}
%
\input{report_front}
\newpage
\input{report_abs}
%\newpage %Uncomment this command if you add a preface
%\input{report_preface} %Uncomment this command if you add a preface
%

l3kernelis too old, and you need to upgrade it to be able to usesiunitx. Don't forget to update MiKTeX in user and admin mode. – Marijn Mar 03 '20 at 13:37\usepackage{ae}which is a bit odd it was a temporary package useful at the turn of last century before scalable versions of the ec fonts were available, it's not often seen now. (Unrelated to your error messages though) also if you are using pdflatex (or even if you are not) delete[dvips]from\usepackage[dvips]{graphicx}the package will select the right option without it being specified. – David Carlisle Mar 03 '20 at 13:58