Can't reinstall MacOs which was deleted by the previous owner

I bought a second hand macbook air, before the old owner hand it to me he deleted everything so i started from "activate mac" page, I can't connect to wifi so I used ethernet, now I am on "restore from time machine, Reinstall MacOs, safari and disc utility" box. I can't reinstall. And now I don't know what to do, can't find anything that will help me from the internet. I hope someone will help. Thank you.