Moses was not allowed to enter the Promised Land because he struck the rock twice instead of speaking to the rock as God had told him to. This is because the rock probably represents Jesus (1 Corinthians 10:4), who was struck once. That is, He was crucified once. To continue, in Numbers 20 we find the account of the Israelites in the wilderness of Zin, and they were without water (v. 2). Moses and Aaron prayed, and God spoke to Moses saying,
"Take the rod; and you and your brother Aaron assemble the congregation and speak to the rock before their eyes, that it may yield its water. You shall thus bring forth water for them out of the rock and let the congregation and their beasts drink.” 9 So Moses took the rod from before the LORD, just as He had commanded him; 10 and Moses and Aaron gathered the assembly before the rock. And he said to them, “Listen now, you rebels; shall we bring forth water for you out of this rock?” 11 Then Moses lifted up his hand and struck the rock twice with his rod; and water came forth abundantly, and the congregation and their beasts drank. 12 But the LORD said to Moses and Aaron, “Because you have not believed Me, to treat Me as holy in the sight of the sons of Israel, therefore you shall not bring this assembly into the land which I have given them," (Numbers 20:8-12)
Moses was told to speak to the rock. Instead, he called the people rebels (v. 10) and said, “shall we” bring forth the water," implying that it was the work of Moses and Aaron to provide for the people. It was not. Rather, it was the Lord who would provide the life-giving water.
As I said above, it could be that striking the rock represented Christ, who is the word of God (John 1:1, 14) and who was struck once (the crucifixion ordeal) out of which living water flowed (John 4:10; 7:38; John 19:34).