Why Avraham avinu did not belive that he could have children? Why was is so if he had strong faith ?

ב"ה Shalom, Again, I refer you to Rashi on Lech Lecha 15:5. It's not that Avraham did not have faith, but Avraham based his thoughts in what he saw astrologically. According to our Rabbis in the Talmud (Shabbat 156a),this is when Hashem educated Avraham that the Jewish people's future is not determined by astrology but by G-d's providence. All the best