updating macOS

After downloading the macOs Mojave from the App Store onto my laptop this message appears when installing. "This copy of the Install macOS Mojave application is damaged, and can‘t be used to install macOS." My computer: MacBook Pro (Retina, 15-inch, Mid 2014) : High Sierra MacBook Pro (Retina, 15-inch, 2014. There is also enough storage space and memory. Why can't it be updated?