Reverted Macbook to Mojave and get an extra unsupported disc alert

Installed Ventura on a 2013 Macbook Pro, worked well but decided to revert to my old Mojave setup. I erased the disc and cloned my Mojave install back but when I restart I get an alert that there some kind of unsupported tiny partition or disc. I want to sell the Mac soon so is there any way to nuke the whole main disc and clone back into there? Thanks