YouTube videos will not play
Why do you link YouTube to QuickTime Player? They are pretty much independent.
error message
What is the exact error message that you get?
Quicktime shows a change made on Jue 9, 2020 that I did not make.
Where do you see this date?
Do you have automatic updates on?
When I click on the Quicktime icon, I get a blank screen. Is this normal?
It is quite normal. Depending on your settings, QuickTime Player may launch with an Open dialog box that has an initial view for iCloud Drive. If you have no QuickTime content saved to iCloud previously, then that initial view will be blank.
To have QuickTime Player’s Open dialog box point to your Mac’s disk instead, change the folder location option in the sidebar of this Open dialog box to a location with QuickTime files on your Mac and open one. QuickTime Player should remember the last used view (folder).