RX 6 is really too old for Catalina (macOS 10.15.7). iZotope had to update their installers back when Catalina first came out a couple of years ago, and RX 6 was one of the products affected, so it sounds like you may have the older, incompatible installer. If you're on Catalina you really should be using a more up-to-date version of RX anyway -- RX 8 is the current version.
Check out this support page on the iZotope website:
And this thread from about six months ago..