reinstall Mac OS to 32bit

Mojave 10.14 runs 32-bit apps; Catalina 10.15 does not. As this is the Mojave forum I assumed you are on Mojave. Am I correct?

I had one 32-bit app that worked with High Sierra 10.13 but did not work when I upgraded to Mojave10.14, where it still should work. It came down to settings for Full Drive Access and Location Services. Once set, the 32-bit apps started working again.

So please a) verify that you are running Mojave 10.14, and b) the name of the 32-bit app that is causing trouble.

You'd be best served to stay with Catalina and download and run the free app GO64 to determine which of your apps are 32 bit and need to be updated and/or replaced. There are good alternatives for most of the apps like LibreOffice for MS Office, etc.

macOS has been 64-bit for 16 years. If you want a 32-bit OS, you'd have to install 10.5 Leopard.

Revert your Mac to a previous macOS version - Apple Support