What makes cooked chicken chewy?
There are two main causes of chewiness in chicken when cooked this way:


Overcooking.   Overcooked chicken is chewy, possibly stringy, and dry. 
Dried out on the outside.  Especially if the skin is removed, the outside may dry out (as well as overcook, even if the inside is not overcooked), leaving a leathery and unpleasant aspect to the chicken.


The solution for the first is not to overcook.  Use an instant read thermometer to know when chicken is done (approximately 155-160 F for white meat, 165 F - 180 F depending on your preference for dark meat).

The solution to the second is to cover, either with the sauce or braising liquid, or aluminum foil or similar so that it cannot dry out.