It is critical that the learner has created his account with his professional email address.
So when you send an invitation to join a team, that invitation is only valid for the email address it was originally sent to.
If your learner was signed in with their personal email address, they are likely to see this alert:
To avoid the conflict between two identifiers, the student must disconnect from the platform if he is connected or registered with his personal email address. Then, he must go back to the invitation email and click again on Accept invitation