Impossible to download Mojave OS to my iMac.

I need to update my iMac to Mojave, but when I use my iMac go to the app store as advised from the site "how to install older versions of OS" I always get the message "this version is damaged and cannot be downloaded." This error message results from something in my iMac. So I thought, no problem, I'll create a bootable disk using my MacBook, which runs Catalina. But when I try to download Mojave from the app store with the MacBook, I get the message that Mojave can't be downloaded because my MacBook is running a more recent version. In short, it looks as if my iMac is ready to throw into the trash. How can I get out of this Catch-22?