To install macOS on a scratch iMac
I am selling my iMac late 2015. I have erased everything to get a new macOS. When I restarted, I got the Apple logo for some time, I see an icon with a file and a "?" on it, and a line saying "support.apple.com/mac/startup". I can't use my keyboard or mouse. What can I do ?
iMac 21.5″, macOS 12.6