I bought a refurbished 2015 MacBook so that I could continue using my legacy software, but its SSD came preinstalled with OS 10.14 Monterey. I want to degrade it to 10.12 Sierra. I have a Time Machine / bootable external HD with Sierra but can't work out how to erase the new SSD and put Sierra on it. I've tried everything, including creating a HFS partition on the SSD, but when I try to run disk utility from the external drive, it doesn't even see the SSD.