@ismail haruna , Thanks for your interest in Azure.
It is recommended that you use a
CNAME for all custom DNS names except a root domain (for example, contoso.com). You can also add/register your URL without the "www." prefix, to set up
another record in order for Azure to verify.
So, for your case (just to show you the domain availability), try without 'www':
Another, test example:
Kindly see this document for more details Add and Secure a Custom Domain on Your Azure App Service Web App.