כִּ֣י אָנֹכִ֥י מֵת֙ בָּאָ֣רֶץ הַזֹּ֔את אֵינֶ֥נִּי עֹבֵ֖ר אֶת־הַיַּרְדֵּ֑ן וְאַתֶּם֙ עֹֽבְרִ֔ים וִֽירִשְׁתֶּ֕ם אֶת־הָאָ֥רֶץ הַטּוֹבָ֖ה הַזֹּֽאת׃
Así que yo voy á morir en esta tierra; y no paso el Jordán: mas vosotros pasaréis, y poseeréis aquella buena tierra.
Rashi on Deuteronomy
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).
Sforno on Deuteronomy
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.
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.