I stumbled across the same issue.
Just continue with the course step 5.3.1 will solve it anyway.
You are probably using the search option for creating a department, that is not allowed because department is non persistable. Just create the object and do any logic after mapping your JSON to the data model.
Looking at your first print screen: select the “Create an object” option instead of the “Find object (by key)” !
I think you have not mapped it.before doing find object ,there was a step instructed to “click on map automatically” which is at top.Try to do that.