Hi Gre,
The error message suggests that the account you're using doesn't exist in the specific Azure tenant, and that's causing the problem.
Here's what you can do to try and fix this:
Check Email: Make sure you're typing in the correct email associated with your old Azure account. Small typos can cause big issues.
Sign Out and Back In: Sometimes, signing out of your account and then signing back in can help refresh things and might solve the issue.
Verify Your Account: Check your email for any messages from Azure that might need your verification. Some actions require you to verify your identity.
Add Your Account to the Tenant: The error message talks about adding your account as an external user in the tenant. If you're using a Microsoft (live.com) account, you might need to somehow link it with the Azure Active Directory of the specific Azure account.
Check Application Settings: While you mentioned looking into the app registration, it's worth confirming that the application with the ID mentioned in the error message is set up properly in the Azure portal.
Try Another Account: If nothing works, you might need to use a different account to contact Azure support. You can even create a new account if needed. They should be able to guide you through the issue.
Give Detailed Info: When you contact Azure support, make sure to provide them with all the details from the error message. This will help them understand the problem better and provide the right solution.
Remember, sometimes these issues can be a bit tricky, so don't hesitate to reach out to Azure support for help. They have the expertise to sort things out.
Please try and let me know if any of this helped you to solve the issue.
Have a great day ahead.