Can't upgrade to 2020

I get a message saying 2020 can’t be opened because it was not downloaded from the App Store on my Mac but what does this mean?
I’ve paid my money but can’t use it and I’m a student
I believe this comes with Layout too correct?
I would love some help please
Many thanks indeed

Which version of macOS?

You may be able to get Gatekeeper to release it by right clicking it in the Finder /Applications and selecting Open. That should give you a dialog where clicking Open will let it run. This only needs to be done once then it will open normally. Not having Big Sur yet, I don’t know for sure whether this still works there.

If that doesn’t work, you probably need to download the installer again and redo the installation, as the developer credentials may have somehow gotten garbled.

Thank you so much indeed I have been able to download it finally Cheers