Hey @Kurt_Ko,
Same error
$ insync-headless account add -a XXXXXXXXXXXX -c gd -p '/home/user/Documents/Google Drive' -e MS_OFFICE
Sorry, an error occurred: (<HTTPStatus.BAD_REQUEST: 400>, b'{\n "error": "redirect_uri_mismatch",\n "error_description": "Bad Request"\n}')
After allowing Insyc to access my account the application name does not show up in the Third-party applications with account access page.