This is a problem faced by me and many others. I have just found out the solution(which worked for me) So I am planning to share it here. 1 - Delete your Teams application. Then Open finder. In while pressing the 'Go' option on the top of the screen, press Option as well. This shows you a hidden folder named 'Library'. Go onto 'Library'.
2 - In the 'Application Support', you will find a folder named 'Teams' And 'Microsoft'. Delete the Teams folder and the Microsoft Folder, delete the teams folder there as well. (Delete Entire Folder).
3 - Go onto Office 365 and login into that. Then go the teams online website and then press the three dots on the top right of your screen beside your profile pic. There you would find a Download for Teams option. Downlaod that. After running the installer, open microsoft Teams. and then (hopefully) you would see the option to sign in from your proper teams Account.
4 - If this works, YOU ARE GOOD TO GO! Try upvoting this so that the one with prolems can find it!