Product distribution file could not be verified. it may be damaged or was not signed.??

• Apps such as Little Snitch, Growl, MacKeeper, and CleanMyMac can prevent apps from downloading
• Disable anti virus software if installed
• Check firewall settings. Open System Preferences > Security & Privacy > Firewall > Firewall Options
• Make sure Gatekeeper is enabled

Open System Preferences > Security & Privacy then select the General tab.

Make sure either Mac App Store or Mac App Store and identified developers is selected.

If that area is grayed out, click the padlock icon to proceed.

OS X: About Gatekeeper

Check your system's date and time setting, and make sure it's correct. (System Preferences -> Date & Time)

App Store apps are cryptographically signed, and these signatures can only be verified if your system's time is correct. If the time is wrong, that causes this verification to fail, and could result in the message you are seeing.

Several causes for this problem have been reported. Please take each of the following steps that you haven't already tried. Stop when it's resolved.

Step 1

If possible, test on another network, such as a public hotspot (if the computer is portable) or the hotspot created by a mobile device. A network gateway that blocks some traffic, most likely on a company or university network, could be causing the error.

Step 2

Back up all data.

From the menu bar, select

 ▹ System Preferences... ▹ Network

If the preference pane is locked, click the lock icon in the lower left corner and enter your password to unlock it. Then click the Advanced button and select the Proxies tab. If any proxy options are selected, make a note of them and then deselect them. You don’t need to change the bypass or FTP settings. Click OK and then Apply. Test. The result may be that you can't connect to any web server. Restore the previous settings if that happens.

Step 3

Rebuild the Spotlight index. If you try to search now from the magnifying-glass icon in the top right corner of the display, there will be an indication that indexing is in progress.

Step 4

Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.

Note: If FileVault is enabled, or if a firmware password is set, or if the startup volume is a Fusion Drive or a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of the test.