Problem downgrading from high sierra to original (shipped) OS
Go to Apple Store and Download OS X

Click on Purchased to see what is available to you

create bootable disk with that OSX

Plug the new bootable installer into your Mac. 


Restart your Mac, holding the Alt key and, when you see the option, choose the bootable install disk.

Launch Disk Utility, click on the disk with High Sierra on it (the disk, not just the volume) and click the Erase tab.

Choose Mac OS Extended from the Format menu, then give the disk a name. `Click Erase`.


When the disk has been erased, restart by holding down Command-R and wait for the macOS Utilities window to appear. 

Choose Reinstall macOS and select the disk you just erased – your Mac’s regular startup disk. Follow the onscreen instructions and wait for your Mac to restart.

When it restarts, ignore the option to migrate data from a backup. 

Optionally --Re-install the backup tool you used to backup your Mac before you upgraded to High Sierra and then use that to restore data from that backup.