Why can't God sin?

[Nobody]

If God has free will, why doesn't he sin?

If we have free will, why do we sin?

Humans can't have free will when one believes they are subjects of a sovereign omniscient being who's scripture informs the reader of them that their lives were predestined before they were born.
Predestination of a life under the authority of a sovereign creator precludes free will.
And a sovereign god can't sin when sin is violating the will and laws of the creator of them. Omnipotence precludes god from being able to sin.

Humans could have been sinless, if one believes the scripture in Genesis prior to the fruit of the tree of knowledge, if god had been omni-benevolent and forgave the very first mistake innocent unconscious humans made, because omnipresent god permitted his agent satan, who only acts by his will, to enter into 'paradise'. Which isn't really paradise at all if the lord of the earth and the god of lies can gain access after the cherubim that guarded the place just so happened to be absent on the day.