If anyone else has this error, it's because you have to enable "Allow apps downloaded from anywhere" on the General tab under Security & Privacy in System Preferences. The problem is that some computers don't have the "anywhere" option anymore, so you have to open Terminal and paste: sudo spctl --master-disable and enter your password. Then reopen System Preferences and the option will be selected. This will work for future apps, too.
After all of this, you'll get the annoying "blocked because unidentified developer" pop-up, which means you go back to the General tab under Security & Privacy in System Preferences and click "Open Anyway".
Yet another instance of Apple trying to save us from ourselves by making us go through steps that make it more likely we'll mess something up. Sigh.