Accessing Microsoft Teams using organizational or personal credentials
Dear Blossom4,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
According to your description, I need to confirm with you that which Microsoft 365 subscription and licenses you had? You could refer What Microsoft 365 business product or license do I have? to check what subscription and licenses you are using. If you find you already had Microsoft 365 E3/E5 licenses and this issue occurs, then we kindly suggest you refer Get support - Microsoft 365 admin | Microsoft Docs to contact our dedicated billing team to get further help, they might available to resolve this issue for you and let you sign-in Teams.
Sincerely,
Tommy | Microsoft Community Moderator