You can probably always put "now" in a present perfect sentence, but in many cases, it will change the meaning. Examples: "I have seen that movie twenty times." This simply means that, up to the present moment, I have seen the movie twenty times. "I have now seen that movie twenty times." This suggests that I just recently watched the movie for the twentieth time (maybe a few hours ago, earlier today, yesterday, or just this moment). There is a strong emphasis on the fact that the statement "I have seen that movie twenty times" has become true just "now." The implication is that it was not true until very recently. Adding "now" to a present perfect sentence will often add this sense that something has only become true recently. "I have now played chess" implies that I had never played chess until just recently (for instance, if I played my first chess game a few days ago, or today). "You have now visited Spain" implies that you had never visited Spain until very recently, but "now" you have finally been there.