In order to save Bnei Yisrael from destruction due to the sin of the spies, Moshe invoked a special trick he learned from Hashem. He preceded his request for forgiveness with a recitation of Hashem’s attributes of mercy (Bamidbar 14:18 with Rashi). We first and most prominently find this formulation after the generation’s other major sin, the Golden Calf (Shemot 34: 6-7). The latter is what we say during selichot (for Nusach Sephard, every day) and is known as the thirteen attributes of mercy. What many people do not realize is that in our parasha around half of the attributes are omitted. Why is that? Obviously, Hashem’s attributes did not change!
The Torah Shleima (Shelach 129) cites a midrash that six of the attributes were off limits because of the lashon hara. The Zohar (III, Shelach 161b) says that "truth" was omitted (rav chesed appears without ve’emet) because the spies lied. However, there is a different approach that it was not that the people did not merit the missing attributes, which are meant, after all, for sinners. The Midrash Aggada (Bamidbar 14:17) says that rav chesed implies that if Hashem would give people what they deserved, no one would survive. Indeed, the Ramban says that this is the reason that Moshe specifically left out emet. He continues that Moshe did not invoke the forefathers’ virtue because they had been promised the Land and here the offspring had turned their back on that present.
The Ramban questions why rachum v’chanun (merciful and compassionate) was omitted. He suggests that Moshe knew that fulll forgiveness was impossible at the time. One can suggest that these were more general matters of compassion that were less focused on forgiving sin, as we say: "Just as He is rachum and chanun, so we should be." It is interesting that even when Moshe chose the attributes to mention, he included "v’nakeh lo anakeh" (I will clean [the slate]; I will not clean), which means that Hashem will only forgive those who repent (Shvu’ot 39a). Moshe’s approach was not to seek full mercy but to allow restitution in a manner that would allow Bnei Yisrael to survive. First the people repented. Second the punishment was to be stretched out over time and not be meted out immediately (see Rashi to Shemot, ibid.). Moshe realized that business could not resume as usual. Thus, Hashem was able to say the famous words, "Salachti kid’varecha" (I have forgiven as you said) and continue that the generation would not merit to see the Land (Bamidbar 14: 20-23).
We have a special right to ask and often receive mercy from Hashem for our personal and national sins. Moshe found a way to tailor-make the request for the need. However, we will not succeed and should not even ask for magical forgiveness that makes sins disappear without consequences or rectification.