The Dertan System contains the Norak Continuum and you do not have a treaty allowing colonization in their systems.
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So you are saying that you cannot colonize in a system containing another empire without a treaty provision allowing it? OR does this only pertain to allies?
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.