Someone in Apple simply forgot to renew a digital certificate, so the entire Mac App Store was shut down or, to put it better, started distributing applications to thousands of users who were unable to settle. 
Software vendors use digital certificates to authenticate the source of the application. They are mainly used for security purposes, allowing operating systems to recognize that an application that is about to be installed in the system comes from a trusted source.
But to avoid any cheating, companies use digital certificates that only apply for a few months or years.
This is exactly what happened on November 11, 2015, when the Apple Digital Certificate, which is automatically added to every App Store app, just expired.
So users who downloaded applications from the App Store after the expiration date acquired applications that could not install them on their computers.
An independent developer spotted the issue, and posted it on Twitter, and of course someone from Apple corrected the problem by issuing a new digital certificate for the App Store.
Meanwhile, Apple continues to be one of the top companies in the stock market, and it is hard to imagine how such issues are.
If you downloaded an app in the last few days and you are experiencing installation problems, you can simply download it again.
https://twitter.com/tapbot_paul/status/664653066679906304
