You can't associate more computers with this Apple ID.

Received message "You can't associate more computers with this Apple ID." on my main MacPro, which is the main computer for all Apple devices. But I was using it this past weekend, so not sure what happened. Just upgraded two phones for my kids, and upgraded to Family plan for Apple Music, could that be it? We have this MacPro, a MBPro, mine & wife & 3 kids phones & 3 iPads & 2 Apple TV. I logged into my iTunes account to "Manage Devices" and there's a generic iPhone listed that's not anyone's, but I cannot remove it (button is greyed out) - it just says "iPhone ( iPhone )". The 5 phones, 3 iPads and only the MB Pro are listed - the final spot is that generic iPhone which I don't know what it is. And now this MacPro is no longer listed. Any suggestions?