Your junction table has no primary key. It does have a composite index though. This means that without a pk, there is no way to uniquely identify a record, so you're getting the prompt. Your tables are not really properly constructed. You should have autonumber pk's rather than use real data as keys. Then the related field in the other table needs to be long data type, not text.
EDIT - forgot to ask why you have put Departments in the relationship twice?
You probably should review db normalization?
EDIT 2 - I for one don't even want to see sub datasheets. Users should not be using tables to view information so they are not really of much use at all. Last, if tables are only for lookups in forms then they don't even need to be inside of the relationships. No harm though, I suppose.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.