After trying a lot of the solutions posted here and they did not work for me, I just called apple support.
And was on the phone for 46 minutes, 17 seconds as they escalated me twice.
They were able to fix the problem, and the fix had nothing to do with my device- they did not tell me what they did, so that wasn't so helpful for everyone here, but it can be fixed.
If you are seeing "Account_unavailable" ... including the underscore- you may have the same issue as me, and you'll probably need to call support to get them to fix it- this page will let you enter a number for them to call you back at.
https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362