The Prayer of Moses

Despite God's refusal to allow Moses to enter the land of Israel, God gives Moses a gentle death and a loving burial.
The Sages teach that Moses had offered 515 different prayers, but one and all were rejected.
I beseeched God at that time Deut.(3:23)

Moses prayed 515 prayers -- the numerical value (gematria) of va'etchanan, "and I beseeched" -- to be allowed to enter the land.

Prayers can only be effective until the final judgment has been decreed; at that point prayers cannot cancel the decree.
Moses' prayers were rejected, because his judgment had been finalized.
Judgement Day is coming!!
'Go on top of the peak and lift up your eyes, westward, and northward, and southward, and eastward, and behold with your eyes, for you will not cross this Jordan.' (Deut. 3:27)
Rabbi Eliezer taught: "Prayer is greater than good deeds, for there was never a man who had better deeds than Moses our Master, nonetheless, Moses was only answered as a result of his prayer, for it says, 'Enough for you! Speak no more to me on this matter' and right afterward it says 'Go on top of the peak...' (Talmud Brachot 32b)

If this is the case, then why would God have told Moses that he need not pray? Rashi in his comments to the Torah addresses the point in the Torah narrative when God says Rav Lach! "Enough for you!" Rashi translates rav lach literally -- rav "a lot," lach, "awaits you" -- and then goes on to explain:

[This means] "there is a lot of good awaiting you," therefore Moses need not pray anymore for his share in the other world.
The answer is subtle, yet simple. God does not change. Man does.
In order for Moses to have a chance to enter Israel, he did not need to fix anything in his relationship with God, his prayers were not necessary. For Moses to enter the land, and more importantly for the people to stay in the land, the people needed to change, to grow closer to the Shechina.
Therefore, Moses is told by God to cease his prayers, and instead Moses is told to take up what he does best: teach.

Moses gives a phenomenal "lecture" in the hope that this will lead his students, his followers, back to God. Moses attempts to fix that which was severed.

In the end, Moses' efforts fell short, but the "lecture" which he left us remains.