[ERROR]Attempt to call nil

print("On/Off")
x = io.read()
if x == "On" then
rs.setOutput("back",true)
end
if x == "Off" then
rs.setOutput("back",false)
end
xcor.clear(1, 1)

function getTree()
while turtle.detect() do
turtle.dig()
print("Digging the block")
turtle.digUp()
print("Digging the block")
turtle.up()
print("Moving up")
end
while not turtle.detect() and not turtle.detectDown() do
turtle.down()
print("Moving down")
end
term.clear()
term.setCursorPos(1,1)
print("Tree Cut!")
end
function cobbleGen()
while true do
if turtle.detect() then
  turtle.dig()
end
itemcount = turtle.getItemCount(16)
if itemcount == 64 then
  for i=1,16 do
   turtle.select(i)
   turtle.dropUp()
  end
  turtle.select(1)
end
end
end
function buildStair()
while turtle.detect() do
  turtle.up()
end
turtle.down()
turtle.down()
turtle.back()
turtle.place()
while not turtle.detectDown() do
  turtle.back()
  turtle.down()
  turtle.place()
end
print("Done!")
end
function buildPortal()
print("Please insert 10 obsidian in slot 1")
print("[ENTER] to continue")
os.pullEvent()
term.clear()
term.setCursorPos(1,1)
turtle.select(1)
turtle.up()
turtle.placeDown()
turtle.turnRight()
turtle.forward()
turtle.placeDown()
turtle.forward()
turtle.up()
turtle.placeDown()
turtle.up()
turtle.placeDown()
turtle.up()
turtle.placeDown()
turtle.up()
turtle.turnLeft()
turtle.turnLeft()
turtle.forward()
turtle.placeDown()
turtle.forward()
turtle.placeDown()
turtle.forward()
turtle.down()
turtle.down()
turtle.down()
turtle.placeDown()
turtle.up()
turtle.placeDown()
turtle.up()
turtle.placeDown()
print("Portal Built!")
end
function clear(arg1, arg2)
term.clear()
term.setCursorPos(arg1, arg2)
end

lights:9: attempt to call nil