The error happens likely because of corrupted caches or iTunes Terms and Conditions not showing up automatically for user to accept in order to proceed. How to fix the error code 100 on Mac App Store? Simply follow the workaround steps below..
How to Fix it
1. Sign out from the Mac App Store via the Store menu.
2. Quit and exit from the Mac App Store program.
3. Log out from Mac OS X user account from Apple menu.
4. Login back into Mac OS X user account.
5. Launch Mac App Store.
6. Sign into Mac App Store.
7. Try to download apps again.
Alternatively, you can try deleting ~Library/Caches and /Library/Caches and then restarting the Mac App Store.