I cannot sign into my school account's Microsoft Teams

I cannot sign into my account; Currently, my brother is currently signed into Microsoft Teams, even though he hasn't used Teams in the past 5 years. It always pops up saying that the account (my brother's) does not have access to view anything in teams and offers a button to sign into another account. When I press that button, it just closes the application (if I am attempting to sign in through the downloaded app on my PC). Signing in using my web browser says that my microsoft account does not exist. I have tried logging into Microsoft 365 using my account (linked to the VSB) and I can access everything (word, onedrive, powerpoint, etc.) but I cannot access Teams, which is most important to me.