Hi @Emmanuel Pineda
Thank you for posting your issue on Microsoft Q&A.
I understand that you are trying to log in to the Microsoft account using a personal account and encountering error that "account needs to be added as an external user in the tenant first".
When you use a personal Microsoft account to log in, you are connected to the Microsoft Services tenant by default, which does not have a linked directory for performing any actions and as you are using personal account you will receive 8-digit code in authenticator app. This behavior is expected. As you are global administrator you can Re-register MFA for user.
If you want to access Entra portal using your personal Microsoft account only, you can invite that user as a guest user as mentioned here: https://learn.microsoft.com/en-us/azure/active-directory/external-identities/add-users-administrator#add-guest-users-to-the-directory and assign the Global Administrator role.
You will receive an invite email once you are added to an azure tenant and accept it, you can use https://portal.azure.com/#create/Microsoft.AzureActiveDirectory URL to create your own tenant as well.
Or you can create a new tenant, open in-private/incognito browser window (just to avoid SSO), access https://azure.microsoft.com/en-us/free/ to create a free Azure account.
When you create a new tenant, you by default become the Global Administrator of the new tenant and have full access to all the options in that tenant.
Hope this helps. Do let us know if you have any further queries.
If this answers your query, do click `Accept Answer`
and `Yes`
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Thanks,
B. Siri Chandana.