Since Catalina, some applications won't launch

Are these apps that you are trying to run 32bit apps. macOS Catalina does not run 32bit apps.

Any 32 bit third party apps you had needed to be checked for 64bit updates from the developers of those apps.

to check for 32bit apps run this, https://www.stclairsoft.com/Go64/

Create a new User account with administrator privileges and see if you can install the apps using that account.

Hey again, that’s good to know, I’d first remove the login items in that user then restart to test:

System Preferences > Users and Groups > Login Items.

I sounds to me more a permissions issue with your user profile, so for that, I’m thinking reset user permissions, / or rebuild the user account.

I know the user permissions repair has changed in Catalina, I’ve found the old way, for me at least, still works, but this article shows the new way to do so:

https://www.google.com/amp/s/eclecticlight.co/2020/03/28/apple-has-changed-resetting-permissions-again/amp/

Hi Heliograms! Sorry for the troubles.

My two favorite tests, (If I don’t already know the specific cause), are testing in a new admin account, as said above), and testing in safe boot:

https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/mac

This will help to isolate the cause to something specific to your user account, such as a setting, preference, permission, and items set to launch automatically. (Another admin user).

And also, (Safe Boot), to see if the problem is intact due to third party/non Apple software that may conflict with Catalina, or even if it was just an issue with the file system). Sometimes, even booting to safe mode and simply restarting again fixes things.

In either case, These tests will at the least tell you if the problem is happening system-wide, or just specific ti your admin-user account, which may help to narrow the cause very much so, so that the cause can be targeted more directly and efficiently. Let us know how it goes!