You might see this error when you try to activate Office for Mac:
The system requires that you sign on to a valid account. The error code is: 0xD000000C
If you've signed in to your Mac using a Network account such as Open directory accounts, this error may occur because Office for Mac can't write your credentials into the keychain as it is 'read-only.'
If you're still having issues with activating Office for Mac, please contact Microsoft support.