What does God warn in the command not to take His name in vain?

Answer: They won't go unpunished

God said the LORD will not leave anyone unpunished (KJV: will not hold him guiltless) who takes His name in vain.

“You shall not take the name of the LORD your God in vain, for the LORD will not leave anyone unpunished who takes His name in vain.”— Exodus 20:7 (BSB)