I have faced a similar kind of issue when I want to delete my demo tenant for some other reasons
Please go to Azure Active directory > users and create a new user in the format @xxxxxx .onmicrosoft.com .
Assign the global admin role to that user.
Now Logon with that newly created user and go to Azure AD > Users. You should see the two user objects
Then remove/delete the external account with which you were logged in earlier.(Ignore if you have not logged)
Once done you can try to delete the directory and this time the directory will get deleted.
The directory deletion information takes around 24 hours to replicate across all the systems the directory deletion information has converged across all related systems within the Azure infrastructure.
Hope you referenced MS article ==>https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/directory-delete-howto
Try from PowerShell also ==>https://learn.microsoft.com/en-GB/troubleshoot/azure/active-directory/cannot-delete-directory-azure-portal
Hope this helps you fix your issue. If you still get any errors please provide the details and I will try to help further. if you feel, you the answer to your questions, please mark this blog as an Accepted Answer
Thanks & Regards,