Can't reinstall Mac OS on 2011 iMac

Thanks for this. Trying to work through but have got stuck...can I trouble you for further help.. I have a 2023 Mac mini which I have downloaded the High Sierra software onto and put it into the Applications folder. I have a Seagate Backup plus hard drive (4TB) and trying to create this as the bootable installer. When copying the command into terminal it gives me an error message 'command cannot be found' as shown in the photo. Is this simple typing error or something else please? I have tried putting in Seagate Backup plus with and without spaces to replace the MyVolume but no success?

Thanks - it won't bring up the utilities screen just gives me an apple logo after recovery. Also worked out that the Installer download wasn't the full download and my attempt to download the full package from the App Store onto a 2015 Mac Book Air failed - said 14 hours - got less than 1 Gb downloaded then hung. Really appreciate the help but think the best option now is to give up trying to install over the internet, or trying to create the bootable installer and instead buy one and see if it will load. Below is a screen shot of the current screen when in Disk Utility - you mention High Sierra not working with APFS. Is there anything I have to do before trying to install a bootable installer please? I am at the limit of my knowledge and don't really understand APFS/Volumes etc and have clearly changed something as it previously used to show MackintoshHD.

Thanks but I am confused. The machine I am trying to restore to High Sierra is the 2011 iMac. I erased the iMac drive but it wouldn't reinstall the software from Disk Utility. I have since tried to make a bootable installer using a Mac Mini 2023 which didn't work as the App Store would not let it download. I then tried to down load the software from the app store onto the 2015 Mac Book air in order to create a bootable installer for the iMac. Non of that worked. The screen shot is of the iMac and all I can do with that at the moment is boot it in recovery mode for High Sierra - which won't then install. Trying to erase the drive to Mac OS extend gives me an error message - 'couldn't unmount disk' 'couldn't communicate with helper application'??

Option-⌘-R just goes to a flashing ? now. I'll try and get a bootable install next and see if that will work.

Thanks - Tried your latest suggestion and got the same message as above.

Contacted Apple Support today to see if they could give me bootable installer. We were able to reboot it in the forced recovery mode for Lion and were able to erase the disk and reset it to Mackintosh HD. Still would not upload the software though - so as they think it must be to do with my internet connection, they are now going to reinstall the software in store for me.

Thank you again for your all your time and help.

Tried again with the correct spelling but still no luck..