@siracusa Hi John, maybe you should call Apple Support. I few years ago I had to change my VISA card and it would not activate in my Apple Wallet. It worked fine for my wife’s Apple Wallet. I spent 8 hours over two days at the local Apple Store and called on the phone before Apple found somewhere in my profile my street address was misspelled. Just my two cents.
@siracusa I think this might be expected behavior to prevent users from abusing AppleCare One. Without this restriction, you could create dummy users on all the Macs in your family that you’re not actively using and add them to the plan. This limitation likely exists to ensure that only regularly used devices are covered. I hope I’m wrong