Hello there,
You have to add a custom domain and verify it with TXT or MX. Before you can add a custom domain name, create your domain name with a domain registrar
Add your custom domain name using the Azure Active Directory portal https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/add-custom-domain
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue. https://learn.microsoft.com/en-us/answers/questions/6955/cant-verify-publisher-domain-for-web-app.html
I hope this information helps. If you have any questions please let me know and I will be glad to help you out.
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