When you attempt a Mac OS X system recovery (boot holding command + r), you get a message this item is temporarily unavailable and it prevents you from completing a re-install of the OS. Some online reports suggest to login to iCloud from Safari by backing out of the installation and selecting Get Help Online from the Mac OS X Utilities screen - but this leads you to a browser unsupported message leaving you dead in the water. Some believe that using a different iCloud account will allow the download of the OS to complete, but there are mixed reports of success with this.
The *ONLY* solution is to create a bootable media drive and re-install Mac OS this way. As long as your machine supports Mac OS High Sierra, you can easily do this.
- Get a USB jump drive or external hard drive and install a bootable copy of Mac OS High Sierra using Install Disk Creator.
- After the copy of Mac OS High Sierra is complete, reboot your mac holding down the option key.
- Select the bootable drive to launch the Mac OS High Sierra installer.
Once the installer is launched, you can re-install Mac OS onto your Mac with the same options as the recovery mode you had before, but without the need to sign in to iCloud.