Check to see if the staff member has any specific characters in their name. At present there is a technical issue when if a staff member's name has an apostrophe, or accent, certain reports can't run. On their staff profile, you can edit their name by clicking on the cog next to their name, then take the apostrophe out of the name. Click Update to save this. If you return to Admin > Config > Tutors you will now be able to assign them to a class. You may wish to revert the name back to how it was, just ensure you remember this for next time you wish to run a staff report.
Our Technical Team are currently working on a resolution, this is just a temporary workaround in the meantime.
Additionally, whilst in the staff member's record, please ensure they are assigned the role of teacher.