I have a 2018 MBP running High Sierra. I need to do some troubleshooting on another older Mac that is running Sierra, so I want to create a bootable USB key of Sierra. Unfortunately my High Sierra MBP won't allow me to run the Sierra installer, saying:
This copy of the “Install macOS” application is too old to be opened on this version of macOS.
Is there any way to create a Sierra bootable USB key, from a machine running High Sierra?