I'm not sure that he has one. In order to have a tragic flaw, you have to be a tragic hero, and in order to be a tragic hero you need to be a fundamentally good character who is brought low by a flaw. One of the oldest debates concerning Paradise Lost relates to whether or not Satan can be regarded as a hero. I would argue that he can't be; the simple reason being that he degenerates morally as the poem progresses. There's nothing remotely heroic about someone who deliberately sets out to corrupt humankind through sin.

However, if after reading Paradise Lost you arrive at the conclusion that Satan really is a tragic hero after all, then you could argue that his tragic flaw is that he becomes too easily misguided. In turn, this is related to his hubris, or overweening pride, inflamed at being cast down from Heaven. But even if you sympathize with Satan, even if you think he has a legitimate grievance against God, it's hard to accept that he goes about seeking justice in the right way, by introducing sin and evil into a previously pure and innocent world.