AppLocker app will not open on Desktop

I purchased AppLocker in the app store and paid for it. The app is in my Applications folder and dock. I click it, but nothing happens. I researched it—the Fire Wall is turned off. I tried updating from the App Store, but the "Unable to Download App" window popped up.

I wanted a password for Apple Messages and WhatsApp on the desktop, as others have access to desktop computer.

A security app that doesn't work makes me insecure. Any input?

Also, I tried to insert a screenshot photo, and it overwrote the copy- lucky I composed the copy everywhere! Huh??