Hello Chris Jones,
Thank you for posting your question in the Microsoft Q&A forum.
When Azure prevents domain purchases or custom domain deployment, the issue typically stems from subscription limitations or configuration conflicts. First, verify your subscription type supports domain services Pay-As-You-Go and Enterprise Agreements typically permit purchases, while Free/Trial tiers may restrict them. Next, ensure your account has Owner or Contributor permissions and check for spending limits that could block transactions. If the domain originates from an external registrar, confirm its availability and proper DNS linkage to Azure.
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