Is the teacher authenticated to their work account? We run into numerous issues with students who have authenticated to a personal Gmail by using Canvas on Chrome when they or another user were logged into the browser with the personal email. I haven't run into this with teachers, but if they've authenticated and shared a doc that lives in their personal G Drive, there could be a disconnect in access somewhere. Have the teacher open Google Drive from the course nav and sign out then reauthenticate. When you reauthenticate you should see what email Canvas is trying to connect to, if its a personal address that should be changed to the work account.
I would also recommend having students open Google Drive from the course nav. If they can do that it's not an issue with their authentication, it's something with the assignment (perhaps the teacher's authentication, or the linked doc is bad)
If you have not already enabled it, Google released the LTI 1.3 several years ago and the legacy LTI (Cloud Assignments) is being sunset in June. Since you are having issue, it may be a good time to transition teachers to the 1.3. Authentication with the Google LTI/Cloud Assignments has been a headache for us over the years. I'm sure we'll uncover more issues with the LTI1.3 as more teachers transition, but I can't say I'm going to miss these authentication hassles from the legacy LTI.