Why was Aaron not allowed into the promised land? God judged Moses and Aaron separately at another event, why wasn't Aaron allowed into the promised land?
Aaron's death was a judgement for what happened at the water of meribah. 


  “Aaron will be gathered to his people because he will not enter into the land which I have given to the children of Israel because you rebelled against Me at the water of Meribah.” (‭‭Numbers‬ ‭20:24‬ ‭MEV‬‬)


The water of Meribah refers to the spring that flowed from the rock that Moses struck. God told Moses and Aaron to speak to the rock to make water flow from it, but instead Moses struck it with his staff. 


  Moses and Aaron went from the presence of the assembly to the door of the tent of meeting, and they fell on their faces, and the glory of the Lord appeared to them. The Lord spoke to Moses, saying: Take the rod, and gather the assembly together, you and Aaron your brother, and speak to the rock before their eyes, and it will give its water, and you will bring out to them water from the rock; so you will give the assembly and their livestock drink. So Moses took the rod from before the Lord, as He commanded him. Moses and Aaron gathered the assembly before the rock, and he said to them, “Hear now, you rebels. Will we bring out water from this rock for you?” And Moses lifted up his hand, and he struck the rock twice with his rod, and plenty of water came out, and the assembly drank, and their livestock. The Lord spoke to Moses and Aaron, “Because you did not believe in Me, to sanctify Me in the eyes of the children of Israel, therefore you will not bring this assembly into the land which I have given them.” This is the water of Meribah because the children of Israel argued with the Lord, and He was sanctified among them.” (‭‭Numbers‬ ‭20:6-13‬ ‭MEV‬‬)


Aaron was called to be a priest and kind of spiritual helpmate to Moses. It was his position to uphold Moses and to support him. Though Aaron did not himself strike the rock, he was there when God commanded them, and he allowed Moses to falter, sharing in his disbelief.