this copy of the install macos mojave application is damage
How to Fix “application is damaged, can’t be used to install macOS” Errors
I just hop into Terminal and use the “date” command to roll back the clock to a date and time closer to when the installer was released. It’s worked every time for me.
For example:
date 020300002018
Sets the clock to 3-feb-2018 at 00:00