Albert Barnes Commentary Deuteronomy 31:2

"And he said unto them, I am a hundred and twenty years old this day; I can no more go out and come in: and Jehovah hath said unto me, Thou shalt not go over this Jordan." — Deuteronomy 31:2 (ASV)

I am one hundred and twenty years old - The 40 years of the wandering had passed since Moses, then 80 years old, spoke to Pharaoh (Exodus 7:7).

I can, no more go out and come in - This means I will no longer be able to go out and come in: that is, discharge my duties among you. There is no inconsistency with (Deuteronomy 34:7). Moses here refers to his own age as likely to make him in the future unequal to the active discharge of his office as leader of the people: the writer of (Deuteronomy 34:1–12), one of Moses’ contemporaries, remarks of him that up to the close of life his eye was not dim, nor his natural force abated (Deuteronomy 31:7); that is, that he was to the last, in the judgment of others, in full possession of faculties and strength.