I have downloaded Catalina, Verson 10.15.4 onto my iMac, but it will not let me use Zoom. How do i fix that?

We have no idea what app you are using where you are referring to Zoom. not working. Please re-write your question with more details so someone can help you. Remember we cannot look over your should and are depending on you to describe in detail so we can get a picture of the problem.

Oh, then you need to be in touch with Zoom's developers, that isn't an Apple product so we cannot help support it.

Are you maybe talking about the Zoom App that people are using to connect during the pandemic? Or do you mean the zoom function to make things larger?