Apple recently pushed (and then remedied) a security update that disabled some users' Ethernet ports. This same Ethernet driver can also affect your ability to launch Mac App Store apps, causing issues for Wi-Fi users, as well.
Mac App Store apps that fail to launch can be a sign of a damaged Ethernet driver.
If you launch an app you got from the Mac App Store and are greeted with the "Sign in to use this application on this computer" dialog, you might be affected. You'll know for certain if, after you've provided your Apple ID and password, you're presented with an error dialog that says, "An unexpected error occurred while signing in. Your device or computer could not be verified. Contact support for assistance." If you get that second dialog you need to apply Apple's Ethernet-related fix regardless of whether or not you intend upon using Ethernet on your Mac.
Most people can fix this by typing "sudo softwareupdate --background" into the Terminal, providing the admin user's login password and restarting. If that doesn't work for you then you'll need to follow Apple's more detailed steps.