attempt to index ? (a nil value)

hello

I'm trying to use a computer to turn my reactor off an on again, but when I ran my program, it said
'attempt to index ? (a nil value)

this is my code

mon = peripheral.wrap("top")
mon.clear()
mon.setTextScale(1)
mon.write("REACTOR 1")

input = read()

if input == "turn off" then
redstone.setOutput("back", true)
mon.clear()
mon.write("turned off")
sleep(3)
mon.clear()
mon.write("REACTOR 1")
end

if input == "turn on" then
redstone.setOutput("back", false)
mon.clear()
mon.write("Turned on")
sleep(3)
mon.clear()
mon.write("REACTOR 1")
end

Line number that the error is at?
Edit: Also this thread needs to be in Ask a Pro section.

oh sorry, I'm new, the line number is 2 I think

I put the coding in the startup bit and it says 'startup:2: attempt to index ? (a nil value)

It looks like the monitor isn't on the top of the computer. In the first line check that the side you put is the one the monitor is on.

it's working now, but the monitor isn't displaying the text and the redstone isn't turning on :/

and do you know how I can program the computer to loop back, so I can turn the reactor off and turn it off again without rebooting?

Here you go:

local mon = peripheral.wrap("top")
if not mon then
  print("Monitor not found")
  return
end

-- function to clear the monitor
local function clear()
  mon.clear()
  mon.setCursorPos(1, 1)
end

clear()
mon.setTextScale(1)
mon.write("REACTOR 1")

while true do -- infinite loop, will repeat the code inside forever (or until you break it or terminate the program)
  local input = read()
  if input == "turn off" then
    rs.setOutput("back", true)
    clear()
    mon.write("Turned OFF")
    sleep(3)
    clear()
    mon.write("REACTOR 1")
  elseif input == "turn on" then
    redstone.setOutput("back", false)
    clear()
    mon.write("Turned ON")
    sleep(3)
    clear()
    mon.write("REACTOR 1")
  end
end

thanks