Cockatiels don't like being petted.
Well mine does. It was hand raised though I guess. Mine bows it’s head down when I put my finger near it’s head because it knows I’ll pet it. Don’t pet under or around the inns though, because even if they seem to like it, it’s s3xual arousal. It all depends on the cockatiels past experiences and if it was hand raised or not. I suggest trying to slowly bringing your finger closer to it’s head in different lengths and reward it if it makes progress, but like the other person said, not all cockatiels want to be petted. Like mine is completely fine with being petted in the head, but does not like being touched on the foot.