In order for incidents to work in Jamf School you will have to create incident types. To do this you need to go to Organisation > Settings > Incident types. Here you can create incident types that you can later use in the creation of an incident.
When you've created incident types these can be used in the creation of an incident. This is done when you select a user and go to the incidents tab:
Here you should see an overview of any incidents related to this user and you're able to register a new incident: