So the Su-57 in this scenario is using in-bay weaponry that consists of only R-77s as far as we know, these are active radar guided air-to-air missiles that require a lock through the airplane's radar to be launched (and even for IR missiles, they would need datalink information from the radar to know where to look in case of a lock-on after launch).
Maverick manages to confuse the radar by flying low and erratically through the mountainous river, this is somewhat realistic, even modern radars can have some trouble in gaining accurate locks in such situations and ranges, and even if it did, the active radar homing seeker would have trouble with a "cold" bandit, which is why in those scenarios IR seekers would perform better.
Now for the question, Maverick's F-14 doesn't have it's radar online (since at that point in the movie Rooster hasn't figured out how to turn his RIO equipment on yet), but he doesn't need it, because he has AIM-9s. These are IR missiles, and since they are suspended weaponry, the sensor is outside and can lock directly from the pylons.
In short, the Su-57 needed a solid radar lock to launch it's missiles, the F-14 didn't.