How to get old versions of macOS
The best this computer can run would be macOS 10.13 High Sierra
To perform this action will require booting from a Bootable Installer. of High Sierra
The Bootable Installer can Only be performed on an Apple Computer
This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.
Example : Bootable Installer of High Sierra would have to be done on a computer that Qualifies to run High Sierra.
Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.
Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive.
It will present options >> Disk Utilities >> View >> View ALL attached Drives.
Choose the Upper Most Drive ( not the volumes indented and list below ).
The drive normally is called Apple Media or Apple SSD
Format the drive as HFS Journaled with the GUID Partition Map
Once that is done >> backup out of Disk Utilities and choose install macOS.
Follow the prompts and it may automatically reboot several time.
