Glad you found a solution.
About the preview, this is HTML math I believe using MathJax. Not all packages and command are known by default. So this \cancel is not recognized by mathjax.
Mathjax support some packages including this ones cancel — MathJax 3.2 documentation
However for the preview it is not easy to setup to make it work.
Easy way is to use
$$
\require{cancel}
\cancel{\binom{n}{s}}
$$
and this should preview ok but no compile in PDF.
To change the mathjax configuration of the preview is not something I know how to do. This would probably be a feature request for the IDE.