Problem with Mac Installer Applications - application is damaged, can not be used to install macOS

[Rod]

krs, my apologies. It seems like every time I look at the App Store they have changed it again. It was always Apple's process to remove the previous macOS when a new one was released and after a time return it on the App Store. It seems this is no longer the case. Apparently you can download the installer again IF you are actually running Mojave or an earlier macOS version, as in the case where you have performed a macOS restore from the Recovery Partition. If on the other hand you have upgraded to macOS Catalina you can't, at least not in the usual way. For this reason I created a bootable macOS Mojave installer on a thumb drive before upgrading to Catalina but it will expire over time and in order to use it I would have to erase my HD first.
Since you actually still have a viable copy of the installer I would suggest following Randy's instructions.

 

[MacInWin]

Rod, this is what works for me. Go to Apple support ( Official Apple Support ) and type in "download High Sierra" or whatever version you want that Apple offered by download. Follow the links.

 

[krs]

I ended up deleting the file named “InstallInfo.plist”.
That should keep the Installer USB Flash drive working forever.

 

[ferrarr]

If you want to see your hidden App Store purchases (macOS, iOS, etc), you need to go into the App Store "account information" page. Different OS versions have different ways to access it.

 

[krs]

Thanks - I found that.

However it says "You have no hidden app purchases"

Weird - because I do have Sierra, High Sierra and Mojave USB installers I made...just in case.
And all these installers came directly from the Apple Store