Why is my macOS Installer Damaged?
If you tried to use a macOS installer in the last few days or weeks, you may have been met with an error message saying something like "This copy of the Install macOS Mojave.app application is damaged, and can’t be used to install macOS."
Verify you have your >System Preferences>Date&Time> Set to Automatic
and try again. (If it is already checked then try unchecking.)
Open Terminal and enter the followign command to set today’s date and time:
101610102014 = Yosemite 10.10
123012122015 = El Capitan 10.11
121401582018 = Sierra 10.12
102510102019 = High Sierra 10.13
102510102019 = Mojave 10.14
Then you can install the MacOS for Mojave