When I log into the Microsoft Teams app, it says that personal accounts are not yet supported on the app.

Did it say something about Teams preview? If so this is a bug. Use this workaround.

Try these steps

First, close and uninstall Teams via Windows settings > Apps.

Log in to the web version, then select "Meet" to generate a meeting link.

Paste the meeting link into a new browser tab and open it; once you see the "Join your Teams meeting" screen, look for the "Download now" option at the bottom and install the application.

After installation, the option to join the meeting will appear immediately; look for the "Sign in" option at the bottom, log in with your account, see if it lets you.

If you have ever linked a work or school email to your PC (even just for Outlook or OneDrive), Windows might be pushing an organizational policy that blocks personal accounts on the Teams client.

Open your Windows Settings and navigate to Accounts.

Click on Access work or school (look for the briefcase icon).

Check if there are any old company, university, or client emails listed there. If you spot one you don't strictly need connected at a system level, click it and select Disconnect.

Open the Windows Credential Manager (search for it in the Start menu).

Click on Windows Credentials, scroll down to find anything related to MicrosoftAccount:user= or IdentityService, and remove them.

Restart your computer, clear the Teams cache, and try logging in again.