@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.
You can then add a CNAME record
for the 'www' subdomain (for example, www.contoso.com).
Follow the steps outlined in this doc '#map-a-cname-record' for the steps.
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.