When your site was a 3.2 it was using an addon-plugin for Google Authentications. 3.5 now has Oauth2 (Google) built into core. To get the 'red' button to show, you might have had to edit a file in the 3.2.
You were using: https://moodle.org/plugins/auth_googleoauth2
Please read the first paragraph.
At any rate, hope you made a backup of code and of the DB prior to upgrading. Your best option at this point is to restore the site to the 3.2.
You will then have to research what one has to do to convert all users that used Google in the 3.2 to the Oauth2 in 3.5 *before* you attempt an upgrade to 3.5 and drop the addon-plugin + setup the new. ** mdl_user ** table will have to be manipulated to get users changed from the old to the new.
There is no utility to make mass conversions other than CSV uploads and those now restrict what fields one can change in the mdl_user table.
Once your 3.2 site is restored, make full site backups ... code, DB, and data directory ... and restore them to a clone server to work through the process of converting users accounts.
'spirit of sharing', Ken