@Khozema Lodhi Further to add:
As its suggested in previous thread that- For Web app this is due to new feature App Service recently rolled out a Name Reservation feature to protect customers from the threat of subdomain takeovers.
This new built-in feature creates a name reservation for web application names.
The name reservation for a given resource name will exist from the moment the resource is created and for a lengthy time period after the resource has been deleted (a cooldown period).
This means that during the cooldown period after resource deletion, other subscriptions outside of the original creating Azure AD tenant will not be able to create a web application using the same name.
However, once the cooldown period has ended, the name reservation will be released, and any subscription will then be able to create a web application with that given name.
Option 1: If possible you can wait for the cooling-off period to expire. (unless it was previously used by an internal subscription)
Option 2: Please reach out to us by sending an email with subject line “Attn:Sneha |thread title” to AzCommunity[at]microsoft[dot]com referencing this thread for help on this.
Please accept as "Yes" if the answer provided is useful , so that you can help others in the community looking for remediation for similar issues.