The people start complaining again despite all God has done for them. This time, they are short of water. Moses makes what seems like a minor mistake of striking the rock instead of speaking to it to get water to come out. It’s not clear why he did that, but God tells him he cannot enter the land God has given Israel because Moses disobeyed him.
Also, if you remember back to the middle part of Genesis, there were twins born named Jacob and Esau. Jacob is the father of the Israelites and Esau is the father of the Edomites. The Israelites expected the Edomites to allow them to walk across their land, but instead the Edomites threatened them.
It’s hard to understand why God punishes Moses so severely for what seems like a minor mistake. Maybe it was because Moses was a leader, and God expected him to set a higher example of obedience. Maybe Moses was trying to look powerful when it was God’s power causing the water to come out of the rock. Either way, we are being reminded that God expects obedience even in the details. When we are trying to accomplish something, we need to do it God’s way.