כִּ֣י אָנֹכִ֥י מֵת֙ בָּאָ֣רֶץ הַזֹּ֔את אֵינֶ֥נִּי עֹבֵ֖ר אֶת־הַיַּרְדֵּ֑ן וְאַתֶּם֙ עֹֽבְרִ֔ים וִֽירִשְׁתֶּ֕ם אֶת־הָאָ֥רֶץ הַטּוֹבָ֖ה הַזֹּֽאת׃
װאָרום איך שטאַרב אין דעם היגן לאַנד; איך גײ ניט אַריבער דעם יַרדן, אָבער איר װעט אַריבערגײן, און װעט אַרבן דאָס דאָזיקע גוטע לאַנד.
Rashi
כי אנכי מת וגו' אינני עבר BUT I MUST DIE [IN THIS LAND], I MUST NOT PASS OVER [THE JORDAN] — Since he was to die, how could he pass over? But he meant: I must die and even my bones will not pass over (Sifrei Bamidbar 135).
Chizkuni
כי אנכי מת בארץ הזאת, “for I am going to die in this land;” this is why I must warn you now not to forget my teachings, as once you have crossed the Jordan I will no longer be with you. If I would have been able to cross with you, I would have been in a position to actively prevent you from forgetting the Torah.
Sforno
כי אנכי מת, I have to warn you very insistently, seeing that I will not be crossing the Jordan with you. Compare Deuteronomy 31,29 where Moses predicts the corruption of the people at some point after his death.