1)
What will not succeed?
1.
Rashi and Ramban #1: It refers (not to the command of Hashem, but) to what the Ma'apilim were about to do.
2.
Ramban #2 (citing Ibn Ezra): Contravening the command of Hashem cannot succeed.
3.
Ramban #3 (according to Kabalah): The Shechinah will not allow you to succeed.
4.
Targum Yonasan: 'Hashem's word (which generally sides with Yisrael) will not side with you, to enable you to succeed'. 1
1
See also Peirush Yonasan and Na'ar Yonasan.
2)
Why would they not succeed?
