Here are some steps you can take to resolve your issue:

1. Make sure that co-teaching is enabled in the Classroom settings.


2. Check if the co-teacher you are trying to add is registered on Classroom.


3. Ensure that the owner of the class (e.g. the Principal or the head teacher) has given the co-teacher permissions to join the class.


4. Verify the co-teacher’s email address is the same as the one used to register on Classroom.


5. Retry the process after a few minutes interval.