This happens even when the user is already logged in globally with their Apple ID. And after login, the app simply closes on its own and repeats the prompt. Users facing this issue report that upon launching iMessage, the app prompts the user to login again. I thought it would be fixed in a patch, but there hasn’t been a patch since Big Sur came out. Even restarting the Mac, and not using it for a few days didn’t fix my issue. I re-installed macOS Big Sur, and it did not resolve my issue. I am having the same problem, after I updated I was not able to sign in at all, I sign in, and about a few moments later it prompts me to sign in again. IMessage sign-in issues after Big Sur 11.1 update Source
