Hi Rajnish,
Please make sure you meet all the requirements:
Prerequisites
- Create an App Service app, or use an app that you created for another tutorial. The app should be in an Azure Public region. At this time, Azure national clouds are not supported.
- To use an App Service domain, the app's App Service plan must be a paid tier, not a Free (F1) tier. See Scale up an app to update the tier.
- Remove the spending limit on your subscription.
https://learn.microsoft.com/en-us/azure/app-service/manage-custom-dns-buy-domain
If this is helpful please accept as answer or upvote.
Best regards,
Dillon Silzer | Cloudaen.com | Cloudaen Computing Solutions