Unable to Detect Operating System

After downloading MacOS Mojave to install via Parallels Desktop 16 on my iMac running MacOS Big Sur, I proceeded to have Installation Assistant to find the disk image automatically. The assistant returned a message saying no disk image could be found. I then manually dragged the disk image on to the Installation Assistant window, where I received the error message "Unable to detect operating system." I then downloaded another copy of Mojave from another site, and received the same result. As I really need to get Mojave running virtually so that I can use a specific software for work that cannot operate in Big Sur, how should I proceed at this point?

 

I have now downloaded MacOS Mojave from four different sources. After the initial failure noted above, today I received the following message in four installation attempts: "This copy of the Install macOS Mojave application is damaged, and can't be used to install macOS." Are all these successive copies of this system truly damaged, or is something going on with my computer?

 

Have you downloaded Mojave directly from apple? https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12 as long as you have iTunes this should triger the ability to download.