וַיֹּ֤אמֶר מֹשֶׁה֙ אֶל־יְהוָ֔ה לֹא־יוּכַ֣ל הָעָ֔ם לַעֲלֹ֖ת אֶל־הַ֣ר סִינָ֑י כִּֽי־אַתָּ֞ה הַעֵדֹ֤תָה בָּ֙נוּ֙ לֵאמֹ֔ר הַגְבֵּ֥ל אֶת־הָהָ֖ר וְקִדַּשְׁתּֽוֹ׃
I rzekł Mojżesz do Wiekuistego: "Nie może lud wstąpić na górę Synai, gdyż Ty przestrzegłeś nas mówiąc: odgranicz górę, a poświęć ją."
Rashi
לא יוכל העם THE PEOPLE CANNOT [COME UP TO MOUNT SINAI] — There is no need for me to warn them, for they have been standing under such a warning these three days, and they cannot go up for they have no permission so to do.
Sforno
וקדשתו, the term is used here as in Exodus 3,5 when Moses was warned not to step on “holy” ground with his sandals as this might prove fatal. Similarly, here; the people had been told that neither man nor beast would survive touching the mountain. [during this period. Ed.]
Or HaChaim
ויאמר משה…לא יוכל העם, Moses said…"the people are unable, etc." What exactly was the distance between Moses and G'd so that G'd had to tell Moses again in verse 24 to descend and warn the people? Why was G'd's previous directive in verse 21 not sufficient? Moses himself tells G'd that there is no need for any new directive, the previous directive having been sufficient to ensure the people would not breach G'd's warning to establish a fence around the Mountain and to sanctify the people.