There should be a chief in your existing population. The one snag is if the chief-to-be is a nursing mother, you will have to wait until she puts the child down, but otherwise, the chief should be there. Make sure you have tried every villager, oldest to youngest. You can go one-by-one thru the details screen, or you can pause the game and move all villagers over to the robe.

Good Luck!

Remember to put spoiler in your subject when asking about puzzles