God is omnipotent. Being omnipotent means you can do anything you want. If God doesn’t have free will, he wouldn’t be very omnipotent. And being omnipotent means he can peer into his own future, to view it in perfect detail. But, if he sees his own future, then his choices are predetermined, and he's not omnipotent. Or, if he can’t see the future, then he’s not omnipotent. Omnipotence is trickier than it seems.
*No, this argument is not me declaring my belief in god, it's me liking the argument that Scott Adams brings up in his book God's Debris.