\usepackage{gensymb} to your preamble, that should enable the command. Another alternative is the\textdegreecommand, which is provided by thetextcomppackage. And, finally,$^{\circ}$is another way of obtaining roughly the right symbol.
For example, in this case, to use the\iintsymbol you need to make sure you have\usepackage{amsmath}in your document preamble, i.e. before the\begin{document}statement. To see the packages Overleaf supports, readWhat packages do you support?
UsingInsert Tableto generate a table for your Overleaf project Please note: this feature is new and more functionality is planned, so keep checking back to this page to see what else is added. We’d also love your feedback on it, which you can provide inthis survey. ...
For example, in this case, to use the\iintsymbol you need to make sure you have\usepackage{amsmath}in your document preamble, i.e. before the\begin{document}statement. To see the packages Overleaf supports, readWhat packages do you support?
This article explores Overleaf’sSymbol Palette—a quick and convenient way to insert math symbols into your document. TheSymbol Paletteis apremium feature. Overleaf premium features are available to participants inOverleaf Commons, members of groupsubscriptions, and owners of individualsubscriptions. ...
For example, in this case, to use the\iintsymbol you need to make sure you have\usepackage{amsmath}in your document preamble, i.e. before the\begin{document}statement. To see the packages Overleaf supports, readWhat packages do you support?