url with # as footnote

Define the url before the footnote with \urldef:

\documentclass[12pt,twoside]{report}
\usepackage{hyperref}

\begin{document}
\urldef\urlchem\url{https://chemicalize.com/#/calculation}

\footnote{\urlchem}
\end{document}

I had a similar problem with Overleaf and an easy workaround was escaping the "#" with "\".

See example:

\footnote{Some constrains apply, see \url{https://developers.google.com/analytics/devguides/reporting/realtime/v3/devguide\#constraints}}.