I followed the instructions on the following article (https://support.apple.com/en-us/HT208969) and get the message "The requested version of macOS is not available." I need this for an older iMac. I erased the hard drive and need to reinstall macOS. Using the control-R, and option-control-R is giving me errors.
The only way I know of to do this would be to have an external USB hard drive*, go into Internet Recovery Mode on your Mojave machine (SHIFT-OPTION-COMMAND-R) so that you can install the oldest version of OS X / macOS available for your Mojave-running Mac and install that as a system onto that external drive.
Then you can boot up onto that older OS X / macOS by holding down the OPTION key.
While booted onto that, as long as it gives you regular-Sierra or something older you can than go into your past purchases in the App Store and download High Sierra.
You can then use that "Install macOS High Sierra" app w/ the instructions to make a bootable USB thumb drive to use on your older Mac needing High Sierra.
How to create a bootable installer for macOS - Apple Support
For ease of use, do yourself a favor and rename that thumb drive "MyVolume" so you can literally cut and paste the terminal command in that help guide.
*Note, instead of a USB hard drive (if you don't have one) you technically could use a 32gb or larger flash drive to install the old OS X / macOS on to. It would just be really slow, but doable. And this is in addition to the 2nd 8gb+ USB flash drive you would need to create the bootable High Sierra drive.
The only way I know of to do this would be to have an external USB hard drive*, go into Internet Recovery Mode on your Mojave machine (SHIFT-OPTION-COMMAND-R) so that you can install the oldest version of OS X / macOS available for your Mojave-running Mac and install that as a system onto that external drive.
Then you can boot up onto that older OS X / macOS by holding down the OPTION key.
While booted onto that, as long as it gives you regular-Sierra or something older you can than go into your past purchases in the App Store and download High Sierra.
You can then use that "Install macOS High Sierra" app w/ the instructions to make a bootable USB thumb drive to use on your older Mac needing High Sierra.
How to create a bootable installer for macOS - Apple Support
For ease of use, do yourself a favor and rename that thumb drive "MyVolume" so you can literally cut and paste the terminal command in that help guide.
*Note, instead of a USB hard drive (if you don't have one) you technically could use a 32gb or larger flash drive to install the old OS X / macOS on to. It would just be really slow, but doable. And this is in addition to the 2nd 8gb+ USB flash drive you would need to create the bootable High Sierra drive.
I'll give it a go!
Downloading High Sierra Installer on a Mac with Mojave
