[Q]

In the Dertran system there is already a colony of the Norak.
You do have a treaty with the Norak.
In your treaty with the Norak there is no element that allows colonization in each other systems.
If it is not allowed you can't do it.

You could colonize in this system if you had no treaty with the Norak or if the treaty contains the specific element that allows colonization in each other systems.