Quarry API

I'm new to programming and need help with this program i am trying to make for the experience(I know there is a built in command, excavate.) but i keep getting the, lua:1: attempt to index ? (a nil value) error. PLEASE HELP

function Quarry()
while turtle.detectDown() do
turtle.digDown()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.turnRight()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.turnRight()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
end
end

Try again after reading the stickies.