Deuteronomy 31:2

וילך משה וגו' ... אנכי היום AND MOSES WENT ... I AM [HUNDRED AND TWENTY YEARS OLD] THIS DAY — Today my days and my years become full: for on this day (the seventh of Adar; cf. Rashi on Deuteronomy 1:3) I was born and on this day I must die (Sotah 13b). לא אוכל עוד לצאת ולבוא I CAN NO LONGER GO OUT AND COME IN — One might think that this was because his physical strength failed him! Scripture, however, states (Deuteronomy 34:7) “His eye was not dim nor his natural force abated!” What then is the meaning of לא אוכל? It means: “I am not permitted” (cf. Rashi on Deuteronomy 12:17 and Deuteronomy 24:4), because the power (leadership) is being taken from me and given to Joshua. — Another explanation of לצאת ולבוא is: I can no more take the lead in the matter of the Law; this teaches us that the traditions and the well-springs of wisdom were stopped up for him (cf. Sotah 13b). וה' אמר אלי — This (according to the previous comment) is the explanation of לא אוכל עוד לצאת ולבוא: I can no more go out and come in because (ו = because) the Lord has said unto Me, [Thou shalt not pass over this Jordan].

וַיֹּ֣אמֶר אֲלֵהֶ֗ם בֶּן־מֵאָה֩ וְעֶשְׂרִ֨ים שָׁנָ֤ה אָנֹכִי֙ הַיּ֔וֹם לֹא־אוּכַ֥ל ע֖וֹד לָצֵ֣את וְלָב֑וֹא וַֽיהֹוָה֙ אָמַ֣ר אֵלַ֔י לֹ֥א תַעֲבֹ֖ר אֶת־הַיַּרְדֵּ֥ן הַזֶּֽה׃

And he said to them, I am a hundred and twenty years old this day; I can no more go out and come in: also the Lord has said to me, Thou shalt not go over this Yarden.