Sforno on Deuteronomy 31:2/ספורנו על דברים ל״א:ב׳

1בן 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.

2לא 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.

3וה' 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.