What will not succeed?
Rashi and Ramban #1: It refers (not to the command of Hashem, but) to what the Ma'pilim were about to do. 1
Ramban #2 (citing Ibn Ezra): Contravening the command of Hashem cannot succeed.
Ramban #3 (according to Kabalah): The Shechinah will not allow you to succeed.
Targum Yonasan: Hashem's word (which generally sides with Yisrael) will not side with you to enable you to succeed. 2
Why would they not succeed?
Ramban and Seforno: Because Hashem had stated adamantly that they were not to go to Eretz Yisrael. Consequently, if they would go against the word of Hashem, they would be contravening His command. 1
Seforno: Not just le'Te'avon (for personal gain) but le'Hach'is (to anger Hashem).
Why did Hashem not accept their Teshuvah (See Devarim, 1:41)?
Seforno #1 (on 13:2): Because of the Chilul Hashem that they had caused. 1
Seforno #2 (in Devarim, 45): Because it was a decree accompanied by a Shevu'ah which Teshuvah cannot remove.
Seforno #3 (Ibid.): Because their belated Teshuvah was based purely on their fear of punishment, 2 it was insufficient to have the decree repealed in this world.