Are you referring to downloading and installing from the App Store. If you're having problems with the App Store you can try this:
1 - enable the Debug menu in App Store:
- open the Terminal application.
- copy the command below and paste into the Terminal window:
defaults write com.apple.appstore ShowDebugMenu -bool true
followed by Enter.
- launch App Store, login.
2 - select the Debug ➙ Reset Application option:
3 - try again.
Can you post the exact error message?