Why can't I win a domination victory in Civilization 5?
Solution 1:
Onondaga
The Iroquois capital city will be named Onondaga. If it is on the map and you have it captured then this is a bug.
If you have not captured Onondaga yet, then you must find it. I would say for certain that it is a bug but your screenshot above doesn't include a picture of Onondaga, so I'm going of the information that is available to me.