"Error 400: invalid_request" on my iMac

The mail app on my iMac can't sign into my main gmail account. I have two other gmail accounts for which all of my email is there and there are no problems. My main email account has the "not connected" icon next to it, and when I try to bring it online I get the Authorization Error message with the code Error 400: invalid_request. I tried removing the account to re-add it, but anytime I now click the Google domain in the internet accounts box, I get that same error message. Please help! I don't have any email problems on the laptop or my phone or on the gmail site, but I believe this error is contributing to other issues because I'm having text message errors related to this as well. Full disclosure, this is a mid-2011 Mac that can't have the iOs updated anymore.

[Re-Titled by Moderator]