Mac App Store Errors
Fix Mac App Store 'An unknown error occurred (100)
Method 1 - The easy route
1. With the Mac App Store running on the screen with the aforementioned error pointing right at your face, simply force close the Mac App Store by pressing the Command + Q key combination on the keyboard.
2. Wait at least 10 or more seconds before you launch the Mac App
Store again.
3. Download or purchase an app, and things should be back to normal.
4. The above method didn't work for you? Then carry on reading.
Method 2 - Signing out and signing back in
1. Launch the Mac App Store and click on the 'Store' button in the Menu bar.
2. Now click on 'Sign Out.'
3. Now click on the 'Store' button in the Menu bar again
and click on 'Sign In.' Enter your password when you're prompted for it.
At this point, things should start working without a problem. But as a
full-proof measure, we highly recommend that you force close the Mac App Store
by pressing the Command + Q key combination on the keyboard and launch it back
again.
The Mac App Store, at times, especially when Apple releases something big, like a new version of OS X, can act up in ways that one can't imagine on a normal day. And while the 'Error 100' message can be a nightmare in situations when you want to download something you've been wishing for a long time, it's best to try out the above two outlined methods before you completely call it quits.
There are instances when the Mac App Store will throw other error
messages at the user as well if you have a choppy Internet connection at hand.
If that's the case, we highly recommend that you run required diagnostics
beforehand to make sure everything is as stable as possible.