Hi @Roger Walters ,
Thanks for the question and using MS Q&A platform.
High payment tiers are constraints on the purchase of a custom app domain even with an available credit card, previous transactions, and a Pay-As-You-Go subscription.
Confirm whether your subscription permits the purchase of an App Service domain. Some tiers have restrictions on the number of domain names that can be created. You might be blocked from purchasing a domain due to insufficient payment history or billing support issues, even with a Pay-As-You-Go subscription. You should also ensure that your credit card is still valid and is properly configured to your Azure subscription. Expired cards will prevent you from completing the purchase. If you are not the account holder, make sure to check with them first as they might have to add additional constraints on domain name purchase.
Reference:
Buy an App Service domain and configure an app with it
Troubleshoot domain and TLS/SSL certificate problems in Azure App Service
Let me know if you have any assistances.
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