Why did my iPhone lock me out?

I bought a new phone and am now trying to sell my old iphone 4. I reset the phone and restored the factory settings, wiping it clean of everything. Today I turned the phone on and tried to unlock it but it’s telling me to enter a pass code. I’ve never had a pass code before and it keeps locking me out. I’ve tried every 4 digit pin I can think of but it just keeps locking me out for longer each time. Please help! I have someone coming to buy it tonight and need this issue fixed!