Mac OS not updating to Catalina 10.15.7

Are you updating the system, i.e. running a version of Catalina now, or upgrading from an earlier system?

If it's the former download and apply the macOS Catalina 10.15.7 Combo Update.   

If you are still having problems boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and and run the Combo updater from there.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

... and have you tried all of the steps in the article that I provided you? If so, then I suggest that you contact Apple directly by using the Contact Support link in the upper righthand side of this forum's web page for further assistance.

How to fix a Mac that won't finish a macOS update - Macworld