Trying to update my MacOS from High Sierra 10.13.6 to Mojave 10.14

When I go to Apple store and attempt to down load it I get a warning window that says: "This copy of the Install macOS Mojave application is damaged, and can't be used to install macOS."

No other Mojave install will work as they all want OS 14 or higher to install to. My mac is a late 2012 w/ 8GB memory so I just need the installer for Mojave OS 14 to be available.