Moses was barred from the Promised Land due to an incident in the Desert. The Israelites were wandering in the desert, and had no water. Basically they started complaining against God. God told Moses to speak to a particular rock and it would give water. Instead, he got shirty with the Israelites and struck the rock, and said something that didn't glorify God. For that disobedience, God banned him from seeing the Promised Land. See Numbers chapter 20 in the bible for the full story. If you don't have a bible, go to www.studylight.org and do a search on Numbers 20.
Moses never entered the Promised Land for the reason that when he was told to strike the ROCK in order for God to provide Water, he STRUCK THE ROCK ''TWICE'', of which meant that he Lost Faith in God for three seconds!