Error 400 with existing gmail
I have always had all my gmail accounts on mail on my mac. Suddenly one of the accounts reports error 400. I have removed it from mail and am unable to add it back as I am constantly told the password is wrong - but it isnt. The only way I can access this gmail account, now, is via my iphone or via gmail itself. Please could someone help me. Thank you.
MacBook (2017)