The problem is Apple keeps making application breaking changes to their operating system. Since 5.5.0, Apple began forcing a new file system called APFS which has incompatibilities with some applications including Unity. The fix for Unity came out shortly after in a patch to 5.5.2. This may be the issue you are seeing if you are having difficulty with drive access. (but don’t go run off and download it, as it still won’t work for Catalina…)
But then with the release of Catalina, Apple decided you should no longer be allowed to execute any 32 bit code anymore. At the time some components of 64 bit versions of Unity were still implemented with 32 bit code, and all of that broke. This happened long after the Unity 5.x release cycled ended, so it will never get the fix. The oldest version of Unity you can use on Catalina is 2017.4 LTS after the fix was added.