Hello Achille Joel Nkwankam,
Thanks for your question.
Before you can create an App Service domain, the subscription needs to have sufficient history on Azure. This may be why you are having this issue with a pay as you go account.
Also To use an App Service domain, the app's App Service plan must be a paid tier and not Free (F1). See Scale up an app to update the tier. Confirm your app service tier.
See reference here:
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Regards,
Abiola