As
@JeroenBurgerhout is mentioning, it's very important that the device isn't managed first of all.
Another thing to keep in mind is that you can't enroll your device in two different tenants with MAM.
Once that's assured, when you enrolled the device, did you remember to uncheck "Allow my organization to manage my device"?
What does the sign-in logs in Entra say? Have you checked the MAM log?
🙂I have 2 blogs around this topic at the bottom of each, where you can find useful troubleshooting methods and other information.
https://www.learnintune.net/byod-the-fun-begins-with-mam/https://www.learnintune.net/byod-the-next-step-with-mam/
