Deuteronomy 30 says God's command is not in heaven or beyond the sea, but where?

Answer: In your mouth and heart

Moses said the command was not too far off: "the word is very near you; it is in your mouth and in your heart, so that you may obey it."

“For this commandment I give you today is not too difficult for you or beyond your reach.”— Deuteronomy 30:11 (BSB)