Why am I receiving an error message when trying to invite co-teachers to my Microsoft Classroom?

1. Ensure that the co-teacher has an active Office 365 Education account.
2. Check the spelling of the email address for the co-teachers.
3. Check if the co-teacher has already been invited to the same class.
4. Ensure that the co-teacher has not already accepted the invitation to the class.
5. Check that the co-teacher is part of the same education domain as the teacher.