VS code CSS class selector not found with angular and bootstrap 5 beta

Uninstall this extension named "HTML CSS Support" and then reload. It worked for me, I hope it would work for you as well.

I had the same problem. It turned out to be an issue with the same 'HTML/CSS Support' extension mentioned in this answer and this answer. It looks like there have been 7 different releases for that extension in the last few days. v1.3.3 seems to be the latest working version of the extension.