This can happen when you use a personal Microsoft account to access a meeting hosted by organization account. The organization settings need to specifically allows meeting to be accessed by personal Microsoft account in order for you to join, so it is a settings on their end that need to change.
However, you can do it this way in the future to join the meetings.
Paste the meeting link in browser incognito mode. When prompted to use app, don't. stick with web.
When prompted to sign in, also don't. You will be let into meeting that way.