I wan't to install Mojave (up from High Sierra) but the link from page https://support.apple.com/en-us/HT210190 is broke; leads to a file that says cannot be installed. Not helpful. Suggestions?

Hi Fyip,

That didn't help, I'm sorry. The link Get macOS Mojave was what I used in the first place with the error message I report. I have not tried the Recovery method, but won't it simply go to the recovery partition and reinstall the system that is stored on my recovery partition, ie High Sierra? Anyway, I'll give it a try.

I tried the How to reinstall macOS from macOS Recovery - Apple Support option, but as I suspected, once I rebooted I could either reinstall what I already have, or install the most recent compatible version: Catalina, which I don't want.

I just now realised that I I had the Update to Mojave app in Applications (which I got when I installed High Sierra), so I deleted it and tried again using the App Store link. It looked like it was working, but after some time loading the file I go the same message, "This copy of the Install macOS Mojave.app application is damaged and can‘t be used to install macOS." Looks to me like the version on App store really is broken.

Eau Rouge solution seems to have worked. Turned off everything so the machine was otherwise quiet, and went via the iTunes/App Store link. Thanks folks for your suggestions.