How to Fix “application is damaged, can’t be used to install macOS” Errors
If you have attempted to use a MacOS installer application recently, you may discover an error message stating something like “This copy of the Install macOS Mojave.app application is damaged, and can’t be used to install macOS.” This prevents the installer of macOS from working and running, and essentially makes the installer applications useless.
The cause of this error is an expired certificate, and because the certificate is expired the “Install macOS” app for Mojave, Sierra, and High Sierra will not run. Fortunately, there is a fairly simple solution to the “damaged” installer problem.