Nowhere in quest info is said you need to kill 20 warrior units (knight, horseman, assassin) on Debir island. Go to Scarlet wind, Rusty anchor . . . when you get enough killed units (and finish some other quest, gather gold, more lds . . . ) go back to Debir and take reward and open skill in hero window.
As for counter itself it must be bug, or it`s just some error try to find some battle that have more then 20 those units and counter should go from 0/20 to 20/20 (in gray color, like finished quest) and there shouldnt be any problems.
Good luck
