I recently did a clean Win 11 home 21H2 build 22000.856 install, and everything went well except for the windows security "action needed" notification. When I opened Win Security, nothing needed attention; however, I noticed the App and Browser Control section was missing. I typed in "App" in the Windows start menu, and the "App and Browser Control" section in system settings popped up. I clicked on it and turned on the "Reputation-based protection," and the "action needed" warning disappeared.
I had the exact same issue and following these steps and turning on "Reputation-based protection" immediately fixed the warning message. Now have a green tick and hover shows "Windows Security - No Actions Needed.".
Thanks for the information, this had been bugging me for weeks!
I bet you are still missing the App and Browser control from "View security dashboard", right? π