The promised land and where He was at the transfiguration are entirely two different things. The promised land is literally the land that GOD promised to give the Israelites. At the transfiguration, it was a divine event that glorified GOD in Jesus Christ and the fact that Moses was there is an obvious sign that he was in Heaven with GOD as well. GOD never told Moses he would not enter Heaven.
Deuteronomy 27:2-4
2 And it shall be, on the day when you cross over the Jordan to the land which the LORD your God is giving you, that you shall set up for yourselves large stones, and whitewash them with lime. 3 You shall write on them all the words of this law, when you have crossed over, that you may enter the land which the LORD your God is giving you, ‘a land flowing with milk and honey,’ just as the LORD God of your fathers promised you. 4 Therefore it shall be, when you have crossed over the Jordan, that on Mount Ebal you shall set up these stones, which I command you today, and you shall whitewash them with lime.
Numbers 20:23-29
23 And the LORD spoke to Moses and Aaron in Mount Hor by the border of the land of Edom, saying: 24 “Aaron shall be gathered to his people, for he shall not enter the land which I have given to the children of Israel, because you rebelled against My word at the water of Meribah. 25 Take Aaron and Eleazar his son, and bring them up to Mount Hor; 26 and strip Aaron of his garments and put them on Eleazar his son; for Aaron shall be gathered to his people and die there.” 27 So Moses did just as the LORD commanded, and they went up to Mount Hor in the sight of all the congregation. 28 Moses stripped Aaron of his garments and put them on Eleazar his son; and Aaron died there on the top of the mountain. Then Moses and Eleazar came down from the mountain. 29 Now when all the congregation saw that Aaron was dead, all the house of Israel mourned for Aaron thirty days.
Matthew Henry's Commentary
The people come to Zin, They murmur for water, Moses directed to smite the rock, The infirmity of Moses and Aaron.
After thirty-eight years' tedious abode in the wilderness, the armies of Israel advanced towards Canaan again. There was no water for the congregation. We live in a wanting world, and wherever we are, must expect to meet with something to put us out. It is a great mercy to have plenty of water, a mercy which, if we found the want of, we should more own the worth of. Hereupon they murmured against Moses and Aaron. They spake the same absurd and brutish language their fathers had done.
It made their crime the worse, that they had smarted so long for the discontent and distrusts of their fathers, yet they venture in the same steps. Moses must again, in God's name, command water out of a rock for them; God is as able as ever to supply his people with what is needful for them. But Moses and Aaron acted wrong. They took much of the glory of this work of wonder to themselves; as if it were done by some power or worthiness of their own. They were to speak to the rock, but they smote it.
Therefore it is charged upon them, that they did not sanctify God, that is, they did not give to him alone that glory of this miracle which was due unto his name. And being provoked by the people, Moses spake unadvisedly with his lips. The same pride of man would still usurp the office of the appointed Mediator; and become to ourselves wisdom, righteousness, and sanctification, and redemption. Such a state of sinful independence, such a rebellion of the soul
against its Saviour, the voice of God condemns in every page of the gospel. (Nu 20:14-21)
