Screen mirroring is not working.

It appears that screen mirroring in Big Sur (11.2.1) conflicts with Norton Anti-Virus. When I uninstall my version of Norton Antivirus - screen mirroring to my Apple TV device works just fine. When I do a fresh install of the app, screen mirroring does not work (except when in the Music app or the QuickTime Player app. So I am running with out antivirus protection. Will contact customer service at Norton.

Yes, the culprit is Norton AntiVirus. The Norton people are aware and working on the issue. A temporary work around is to temporarily turn off "Vulnerability Protection" which you can access from the Norton Antivirus App under the Advanced/Firewall settings. Turn the protection back on when thru with screen mirroring. Note: you don't have to restart to turn off/on the Vulnerability Protection.

Thank you for your reply. Looks like you didn't consider these points in my issue: "But when in Music or QuickTime Player, I can project videos thru my Apple TV to the TV screen. Also, from the "Apple TV" app on my Apple TV, I can pull videos and recordings from the library on my MacBook. Note that if I boot to Mojave on an external HD, screen mirroring works just fine!"

The issue of non-connection occurs when in Finder, Safari, Zoom, etc., and I attempt to "screen mirror" via the "Displays" function in System Preferences. Therefore connection failure is not an issue of router settings (Apple Time Capsule), interference from other devices, insufficient signal strength, etc. It appears to be a software or system settings issue. Please advise.

Had about a two-hour chat with reps from Apple's Customer Service. We went through all the motions of getting the screen mirror function to work while in Finder, Safari, etc. No avail. We ended up generating and collecting system diagnostic data from both my MacBook and Apple TV. These data were forwarded to Engineering for analysis and hopefully a solution. Stay tuned.