You can't create another partition because the rules state that:
=> Only 3 primary partitions can be created if there exists an extended partition.
=> 4 primary partitions can be created if you don't have an extended partition.
As your screenie shows, you already have 3 primary & 1 extended partition.
You should have created 1 primary partition and 1 extended partition. then you could have had logical drives in that extended partition.