The Posuk (Devarim 3:26) says:
רַב לָךְ אַל תּוֹסֶף
What is the signifigance of this language?
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The Gemara (Sotah 13b) says that it parallels Moshe's statement to Korach and his group, "רַב לָכֶם בְּנֵי לֵוִי". In return, he was told "רַב לָךְ".
Maharal, in Chiddushei Aggados, explains that Moshe was, in effect, telling them, "The greatness that you have ought to be enough for you - you shall not have any more." Which is a degrading way to talk to them. Because of this, then, Hashem used the same wording with him.
Ben Yehoyada takes a slightly different tack. He explains that even though Moshe had to be firm with them and deny the validity of their arguments, he still did so as respectfully as possible, telling them "רב" rather than the more dismissive "די". Accordingly, when Hashem had to deny Moshe's request to enter Eretz Yisrael, He did the same thing.
The Mahram Shif answers that Moshe requested that he go into Ertz Yisroel even if Yehoshua would be his Rebbi. Here Hashem is responding cant do it. You want to accept Yehoshua as a rav do not speak any further of it I will not accept it. That is about Yehoshua as your Rav