@Brian K Thanks for asking question! You may want to know that App Service Domain recently introduced a new policy to require sufficient payment history prior to purchasing a domain. Without any payment history, your subscription will not pass the check that will allow your subscription to purchase any domains.
Could you see if you have any pending payment due or due date past on that subscription. you can follow this article to check invoice: View and download your Microsoft Azure invoice
For questions related to billing or subscription, you may file a free subscription/billing support ticket, our support engineers will be able to help on such issues readily.
create a support request – You will receive swift and specialized assistance on the from the support.
Having said that reached out to you privately, Let us know.