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This can happen for several reasons.
- Some of the Azure Subscription types don’t support domain purchases, including Free, Sponsorship, credit-based subscriptions, CSP, or certain Enterprise Agreements.
- If it's a CSP or restricted subscription, you must purchase the domain externally (e.g., GoDaddy, Namecheap) and manually configure it.
- https://learn.microsoft.com/en-us/azure/app-service/manage-custom-dns-buy-domain
- Remove the spending limit: For credit-based subscriptions, removing the spending limit can enable the purchase of App Service Domains. Be aware that this action may lead to charges beyond your included credits. Read more on the Q&A below1. https://learn.microsoft.com/en-us/answers/questions/2033578/creating-app-service-domain-with-credits-available
- Sufficient History: For some subscription types, before you can create an App Service domain, the subscription needs to have sufficient history on Azure
You can purchase the domain externally and map it to the App Service in any of the scenarios.
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