Can God make square circles? If not, is he not all-powerful?
The short answer
God can do all things that are possible. He cannot do what is impossible for him — sin, or logical impossibilities like square circles. That is not a limit on his omnipotence; his power simply does not contradict itself.
All things possible
God can do all things that are possible (Matthew 19:26), including what is impossible for man (Luke 18:27).
What God cannot do
He cannot do what is impossible for him: sin, or create logical impossibilities such as square circles, mathematical errors, or a rock too heavy for him to lift.
Still omnipotent
This doesn't mean God is not all-powerful. It means his power does not negate itself.