Unable to join Teams video Meeting Macbook

It sounds like Teams is automatically using your old account, which is not permitted by the meeting organizer’s policies.

Open the invitation link in an Incognito/InPrivate browser window, stay in the browser, and select Join without signing in. If available, select Change and use the account that received the invitation. You can also try the meeting ID and passcode from the Gmail invite.

Calendar syncing will not necessarily add the meeting to the Teams calendar, but you can still join from the original invitation. If anonymous joining is disabled, the organizer must allow external participants or involve their IT administrator.