Thank you for your post and I apologize for the delayed response!
I understand that you're trying to remove an old Azure AD tenant/ directory, so you just have your default directory but are having issues deleting/removing the old one.
When it comes to Deleting a tenant in Azure Active Directory, please make sure that the user you're logged in with is a Global Administrator, and that all the checks pass when you're trying to delete the tenant. For more info - Prepare the organization.
Note: These checks reduce the risk that deleting an Azure AD organization negatively affects user access, such as the ability to sign in to Microsoft 365 or access resources in Azure.
- Sign in to the Azure portal with an account that is the Global Administrator for your organization.
- Select Azure Active Directory.
- On a tenant's Overview page, select Manage tenants.
- Select the checkbox for the tenant that you want to delete, and then select Delete.
- If your organization doesn't pass one or more checks, you'll get a link to more information on how to pass. After you pass all checks, select Delete to complete the process.
I hope this helps!
If you have any other questions, please let me know. Thank you for your time and patience throughout this issue.
Additional Link:
- Troubleshoot Tenant Deletion - Documentation for the different steps to troubleshoot tenant deletion.
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