First, I don't have 15.1 installed as I don't install beta's on my primary devices and my M1 MBP is the only supported Mac I have for AI. I would first request to make sure 15.1 is indeed installed. You never know, it's part of that never assume a computer is plugged in part. I would then state that in Settings you should see an option for, "Apple Intelligence & Siri". There is a toggle to turn it on but from what I hear, there is a waitlist to be able to start installing the AI part. Not sure how the waitlist works but don't expect it right away I suppose.

https://support.apple.com/en-us/HT210904