QuickTime

Is this about QuickTime Player X (10.5) or QuickTime Player 7 (7.6.6)?

QuickTime Player 7 is no longer compatible with macOS 10.15.

Two tests for you. Please let us know how it goes:

• Use safe mode to isolate issues with your Mac - Apple Support. This should help find out if other software is interfering.
• How to test an issue in another user account on your Mac - Apple Support. This should help find out if the issue is with the system software or with user preference files.

You can’t open the QuickTime application;

or: You can’t open a QuickTime movie?

If you can’t open a QuickTime movie, then please let us know what the program Invisor Lite reports when it inspects your movie file.