Kittens game assigning leader bug
Solution 1:
From your screenshot, you actually are able to design a Leader.
In the list of kittens, locate the one you want. At the right-hand side is a white star. (Above the "Unassign Job" mention). Click on the star. It turns black : your kitten is now a Leader. He / she is now featured above the kittens list.
To change Leaders, just click on the star next to the future leader's name.