Vehicle Inspection New Reservation System

I got a notice it's time to do my vehicle inspection test.
They said now you have to make reservations so I went online
to the  website they sent me: www.cyberts.kr/yeyak

and I had to input my vehicle registration number and then the first 6 digits of my
resident number.

I tried but it keeps telling me my vehicle registration number is not correct.
What is a vehicle registration number?  Is there a translation error because
I searched online and they said it means license plate in korea and then another place
said it means your VIN number.  So I tried both license plate and VIN number and both
times it was incorrect.

I think they are referring to another number that identifies the car but I have no idea
what they mean by vehicle registration number and where you would find that.

Is there no registration number on the notice they sent you? It's probably related to the car tax registration. I can't remember sorry, maybe ask some Korean friends or co-workers who have cars.