If a staff member has a special character in their name, this may prevent certain activity in ScholarPack reports. On their staff profile, you can edit their name by clicking on the cog next to their name, then take the character (e.g. accent over an e) 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.
Additionally, whilst in the staff member's record, please ensure they are assigned the role of teacher.