Problems installing Sonoma on 2015 IMac

I recently acquired a 2015 IMac that has been factory reset and runs on OS X El Capitan. I’m following the Mr Macintosh YouTube video to install MacOS Sonoma and I’m running into an issue where the installer is downloaded but will not let me move further because it says it requires High Sierra first?

I don’t see High Sierra on the OCLP installer list so I’m not sure what to do now. Can someone help?

submitted by /u/ReferenceLanky2084
[link] [comments]