Chicken kebab turned hard
What probably went wrong for the meat to turn hard is that it was overcooked and dried (which happens easily with chicken).

Having said that, you could experiment with reducing temperature/time for better results. Also, a trick I use with chicken kebabs (but I grill them, so this might not work for frying) is that I cover them in yoghurt, which works as a protective layer to keep the moisture inside while the meat cooks.