I bought MacBook Air from a seller , he reinstalled the Mac OS however when I came home , I was given a screen of enter system lock pin code. Is there anyone that can help me solve this.

I went to Apple Store however they said that I must have a receipt of purchase which I don't have and I tried to contact seller however he blocked and didn't answer my phone call.

Prefer KL area or Kota damansara.

I'm not sure but when he showed me it was inside the system like checking all the button , wifi etc...

Anyway to solve this problem ? Apart from letting Apple solve this