Update

If you still have problems, please answer to these questions:

1. What iOS version is currently running on your 12? (Settings>General>About>Software Version)
2. What happens when you try?
3. Are you trying to update it wirelessly or via computer?
4. What error message are you getting?

Regards

Giulio

Well, if the upgrade message pops up, it means that you can perform the update procedure.

Regards

Giulio

Yes, it can. See this compatibility list:

Considering my iPhone 8+ has accepted and generally works satisfactorily, I would find it pretty strange your 12 wasn't currently supported.....