Click on the line and and see how many devices are attached with that number and also try to see how many T-Mo ID s are attached with that number. In either case you cannot have more than 5 devices for a single line and 5 T-Mo IDs for a single line. Similarly, you cannot assign more than 5 numbers to a single T-Mo ID.
Someone from T-Mobile responded elsewhere. Looks like this was a temporary issue with the system on their end that affected more than just my account.
Funny how no one seems to have the REAL answer... yet it is listed on REDDIT...if you switch devices often it piles up in your devices attached...you can go HERE to remove old devices from your number: