OVERLOOKED FACT

No macOS installer will install over a subsequent version macOS In other words if you have macOS 10.12.6 on your system, you can reinstall macOS 10.12.6 or install macOS 10.13 or later but you cannot install macOS 10.12.5 or earlier unless you nuke & pave. Normally the installer will report the error and stop but this probably the reason for your failed install attempts. To go backwards from the currently installed version you must completely erase the drive first, which can only be done from another computer, a bootable thumb drive installer, or bootable external drive. (Note: in Big Sur and Monterey on M1 Macs you need a second computer and Apple Configurator 2)

Last edited by joemikeb; 11/10/21 05:47 PM. Reason: Posting glitch
If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein