Major Problems when resetting to factory settings and wiping machine

Hi guys, I'm in the process of trying to wipe my 2017 macbook air in order to sell it as I've upgraded. Im not super experienced with computers and I'm worried I've made a pretty significant mistake! I followed the recommended steps of signing myself out of everything and restarting the machine then pressing 'command+R'. I followed the youtube video I was watching and wiped the disk of the machine however I was running into an error message "Could Not Create a Preboot Volume for APFS". I once again followed another tutorial on how to get round this and was told to shut down my machine and proceed t then press 'option+command+R+Power Key'. The machine rebooted just like the guy in the videos but as it turned on I was presented by a much newer looking Interface. My machine is now asking me to install Big Sur rather than macOSHighSierra and my disk now comes up as disk0s2. I cant seem to access HighSierra anymore and the machine wont install BigSur either. I know this is a lot of detail but I would really appreciate a hand. Thanks!