Incorrect Date and Time settings

The Mac App Store relies on having the correct Date and Time settings to authenticate its connection. Check to see if the Date and Time settings are correct by taking the following steps:

1. Choose System Preferences from the Apple menu ().
2. Click the Date & Time icon in the System Preferences window that appears.
3. Make sure that "Set date and time automatically:" is selected, and that the selected server is set to your region.
   For example, in North America it should be set to "Apple Americas/U.S. (time.apple.com)".
4. Verify that the correct date and time are displayed in this preference pane.
5. Choose the Time Zone tab.
6. Make sure that the correct Time Zone is selected.  

Missing Serial Number

The system serial number is used for authentication purposes. If the serial number is not displayed in About This Mac, the Mac App Store can't authenticate its connection.

1. Choose System Preferences from the Apple menu ().
2. Click on the version number under OS X in the window that appears. The information should change to a build number.
3. Click on the build number under OS X. The information should change to your computer's serial number.
4. If no serial number is displayed, contact AppleCare for further assistance. 

Invalid or incorrect Network Preferences 

Creating a new Network Location can assist in isolating potential Network Preference issues. 

• To create a new network location, see Using A Network Location. 

Invalid or incorrect Certificates

Use the following steps to reset the cache of accepted certificates.

1. Open the Finder.
2. Choose Go to Folder from the Go menu.
3. Type /var/db/crls/ in the Go window.
4. Click Go.
5. Delete crlcache.db and ocspcache.db by dragging these files to the Trash.
6. Enter an administrator password if you are prompted.
7. Restart the system and test for the issue.

Edit your Keychain.

The Keychain holds passwords and certificates used to create authenticated connections from apps such as Mac App Store. If the issue still happens after following the previous steps, try editing your Keychain.

1. Open Keychain Access located in /Applications/Utilities/
2. Select Certificates from the left hand column under Category
3. Type Class into the search field in the top right corner of the Keychain Access window and press return.
4. Look through the list of results for any certificates that have a blue + over the icon.
5. Double-click on a certificate that has the blue + over the icon to ‘Get info’ on the certificate.
6. Click the disclosure triangle to the left of the word Trust to reveal the certificates permissions.
7. Change Secure Sockets Layer (SSL): to ‘No Value Specified’ then close the window.
8. Confirm the Administrator password in the window that will appear.
9. Double-click on the same certificate that was opened in step 5.
10. Again Click the disclosure triangle to the left of the word Trust to reveal the certificates permissions.
11. Change When using this certificate: to ‘Use System Defaults’ then close the window.
12. Again confirm the Administrator password in the window that will appear.
13. Repeat steps 5 through 12 for any certificates that have a blue + over the icon. (Generally there are only one or two that will need to be edited.)
14. Restart the system and test for the issue.

Important: If the certificates do not allow themselves to be modified as is needed in steps 5 through 11, you need to reinstall OS X. After reinstalling, attempt these steps again.