Log in to the App Store with the user ID and password you used to purchase your Mac. If you're still having problems with the App Store you might 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.
Lastly contact the Support personnel via this link at the App Store: