Deuteronomy 31:2

and said to them: I am the son of a hundred and twenty years this day. I am no more able to go out and come in, and the Word of the Lord hath said to me: Thou shalt not go over this Jordan.

To go to and fro denotes going out to, and returning from, battle. In other words: “Even if I were not now going to die, still I am incapable of waging war. Thus, you have no need of someone who would leave you unattended. But rather, God will destroy the nations, and Joshua also — and the proof is that which you have seen with your very eyes, namely, the war of Siḥon and ‘Og.”

AND HE SAID UNTO THEM: ‘I AM A HUNDRED AND TWENTY YEARS OLD THIS DAY.’ This was to comfort them concerning his condition [i.e., about his approaching death], as if to say, “I am old and you have no more benefit from me. Moreover, G-d has commanded me that I should not go over there. Do not dread and do not be fearful, for the Eternal will go over with you; He will not remove His Presence from you on my account [i.e., because of my absence], and Joshua, he shall go over before you in my place.” Now, although Moses our teacher retained his vigor and health, as Scripture testifies, his eye was not dim, nor his natural force abated,, 34:7. he told them this [“I am old and you have no more benefit of me”] in order to comfort them [over the transfer of leadership to Joshua].
Now, Rashi wrote: “I can no longer go out and go in, because the Eternal hath said unto me: Thou shalt not go over this Jordan.” But it is not correct. (‘and’ the Eternal) said unto me. Now, Rashi explains that the connective vav, which generally means “and,” signifies “because.” It is this interpretation which Ramban considers incorrect because vav always means “and,” not “because.” Mizrachi, too, raised this objection to Rashi and left it unanswered. See, however, in Ma’aravi where a different version of Rashi’s text is cited, in which the connective vav has its usual meaning. And in the opinion of Rabbi Abraham ibn Ezra the expression I can no longer go out and come in means to war, because his powers weakened in his old age. This too, is not correct. (further, 34:7). And our Rabbis have said: “This teaches us that the well-springs of wisdom were stopped for him.” This was a miraculous event in order that Moses should not be troubled [about the transfer of leadership to Joshua] and [G-d] bestowing honor upon Joshua in his [Moses’] presence.

בן מאה ועשרים שנה אנכי היום, Moses mentions his age to remind the people that there is no reason to be sad that someone of his age has to die. According to any life expectancy and law of nature, he was no longer fit to go on living by natural means.

לא אוכל עוד לצאת ולבא, even if I were still able, I would not be able to function physically as a leader should, such as leading you in he conquest of the Land of Israel as I am suffering from symptoms of old age.

וה' אמר אלי לא תעבור, furthermore, G’d had said to me a while ago: “you will not cross;” even if I were physically capable of leading you, the fact that G’d told me that I would not cross the Jordan would have made my leadership pointless. You are therefore better off without me so that you will be able to cross.

בן מאה ועשרים שנה אנכי היום לא אוכל עוד לצאת ולבא, “I am one hundred and twenty years old today, I can no longer go out and return home.” One reason I cannot do so is my age; another reason is the fact that G–d has forbidden me to cross the river Jordan. You will not suffer a setback by my not crossing the Jordan, as G–d, personally, will cross with you, and He will destroy etc.;” i.e. כי ה' הוא עובר לפניך והוא ישמיד וגו'. I do not feel weak, but I must obey G–d’s command.