After the Sin of the Spies, as a punishment, G-d pronounced the sentence of doom on the male population from the ages of 20 -60. Therafter the Torah states, "They ascended the mountain top saying, 'we are ready, and we shall ascend to the place which G-d has spoken, for we have sinned.' "(Bamidbar 14:40)

The Jewish People were embarrassed and humiliated. They finally realized that their reaction to the Spies' slander of Eretz Yisrael was terribly wrong. Moshe told them that their Teshuva was rejected and not to attempt to enter and conquer Eretz Yisrael.
Some of the Isarelites would not accept a negative response and insisted upon going on to Eretz Yisrael. They failed big time. They were attacked and utterly defeated
 by the Canaanites.

Why did G-d reject their Teshuva? They acknowledged and confessed, "We have sinned," which the Rambam says is the essence of Teshuva. The Talmud in Yoma 86 states that Teshuva is so awesome that it can change a past sin into a present Mitzva. Three times a day we pray, "BORUCH ATA HASHEM HAROTZEH B'TSHUVA"
(G-d desires and eagerly anticipates our Teshuva).

The Or Hachaim Hakodosh states that G-d rejected their Teshuva, because it was insincere. Their declaration of Teshuva was inspired by regret over the consequences of their sin in losing Eretz Yisrael, rather than by remorse over their sinful behavior.

The Baal Shem Tov responds to this question with a keen observation about human nature. He punctuates the verse in Bamidbar 14:40 in a manner that changes the meaning. They were actually saying, "We are ready to go to Eretz Yisrael, ASHER AMAR HASHEM KI CHOTONU (for G-d has said that we have sinned)". In other words they did not concede that they had sinned. It was G-d Who said they had sinned! Their smug and self-righteous attitude was obvious. They conceded that G-d was not happy with their behavior, but they did not agree with His perspective. They still remained defiant.

We compound our sin by refusing to acknowledge our sinful behavior. We seek to justify our actions. How can we do Teshuva if we refuse first to recognize and admit our errors? Self-denial leads to an obstinacy that confuses the individual and his perspective.

The Nation of Israel was prepared to fight and attempt to conquer the Land of Israel. They were prepared to do whatever was needed to go up to Eretz Yisrael, everything but to concede the truth that they had in fact sinned.  

The lesson of this sad episode of the Spies is to be truthful and honest with the person in the mirror!