Selecting "About this Mac" will not open: OS Big Sur

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for Avast and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally. 

The SN is on the bottom of the stand and also on the original box.

By the way, do you have any third party antivirus, cleaning or any other maintenance" apps installed on this computer?

The SN is on the bottom of the stand.

Do you have any third party antivirus, cleaning or any other "maintenance" apps installed?

Your welcome! Glad it is working okay now.