The App Store link to High Sierra actually leads to Venture. Anybody have a link that works to get the High Sierra installer?
What year model is the iMac?
see> Identify your iMac model - Apple Support
You can only install the original or a later macOS version.
Go back to the link that Old Toad posted and try Mojave, Catalina, Big Sur or Monterey. Again the earliest macOS version that you can install, hinges on the year model that you have.
A Late 2009 iMac can only run up to High Sierra.
Try starting up in Internet Recovery Mode Option-Command-R install the offered OS X version and upgrade to High Sierra if needed.
Thank. Actually, I just finished a chat with an Apple advisor. It turns out that because I'm trying to download a very old OS on a relatively new iMac, the system won't let me do it. I'll have to do some kind of workaround, or possibly see if I can use an older Mac to download the High Sierra installer and create the USB installer stick.
It would have been nice if the App Store had said "you can't download HS on your current system" rather than linking to Ventura with no explanation.
Late 2009, 27" iMac.
need High Sierra installer to refurbish my old iMac
