It's fairly simple.

Most Christians say God can do anything (omnipotence) and knows everything (omniscience).

It is inherently self-contradictory for something to be both omniscient and omnipotent.

If God knows everything, then he'll know what choices he'll make tomorrow. (example, he knows he's going to choose A instead of B)

If God can do anything, then he can choose either A or B, meaning he could choose B even though his knowledge of the previous day said he would choose A.

Therefore, if God cannot choose B because his foreknowledge told him he would choose A, then he cannot do anything or lacks omnipotence.

And if God does choose B, as an all-powerful being could, then his prior knowledge that he would choose A was wrong, in which case he is not all-knowing or lacks omniscience.