Not all team members are available in the notification list

I’m trying to add an automatic notification that notify a specific team member when a task’s status changes to “Done”. In the “Team member” drop down list in the automation tool I don’t see the team member I’m looking for. Everyone is there except him, although he is invited and can see the content of the board.
What can I do? why don’t all of my team members in the team members list in the notification automation tool?