stuck on checking null
Just bought my 2018 MacBook Pro from someone and he had it ready to download the new os but I stupidly accidentally restarted it and can't get back to that screen. I found OSventura but it keeps getting stuck saying checking"null" and I'm not sure what to do. I've restarted and rebooted many times. Please explain this to me like I'm 5 as this is my first Mac purchase ever.
iPhone 8 Plus, iOS 13