I am having trouble upgrading from High Sierra to Mojave. Any thoughts??

I have a MacBook Air 13 inch, mid 2012 running High Sierra. I want to upgrade to Mojave. The attempts to download the installer from the App Store get the following message. This copy of the Install macOS Mojave application is damaged, and can‘t be used. I believe my Macs hardware will support the upgrade. Any ideas what is causing this and how I can get a valid Mojave installer???