I hope this works this time

local termWidth, termHeight = term.getSize()
local keepRunning = 1
local reactor = peripheral.find("BigReactors-Reactor")
local monitor = peripheral.find("monitor")
local selectedItem = 1
local onMainMenu = true

local input = nil

-- Begin Main menu --

function Choice1()
  term.clear()
  term.setCursorPos(1,1)
  print("Reactor Enabled.\n")
  reactor.setActive(true)
  sleep(1)
end

function monitorOutput()
    -- Define Statistical Values --
    active = reactor.getActive()
    energy = reactor.getEnergyStored()
    fuel = reactor.getFuelAmount()
    maxfuel = reactor.getFuelAmountMax()
    waste = reactor.getWasteAmount()
    reactivity = reactor.getFuelReactivity()
    rate = reactor.getEnergyProducedLastTick()

    -- Begin Display --
    term.clear()
    term.setCursorPos(1,1)
    monitor.write("Reactor Active: "..active..)
    monitor.write("Stored Energy: "..energy.." RF")
    monitor.write("Fuel Reactivity"..reactivity.." percent")
    monitor.write("Total Fuel: "..fuel.." mB")
    monitor.write("Total Waste: "..waste.." mB")
    monitor.write("Total Contents: "..fuel+waste.." mB of"..maxfuel.." mB")
    monitor.write("Energy Per Tick: "..rate.." RF last tick")
  end
end

function Choice2()
  term.clear()
  term.setCursorPos(1,1)
  print("Reactor Disabled\n")
  reactor.setActive(false)
  sleep(1)
end

function Statistics()
   -- Define Statistical Values --
  active = reactor.getActive()
  energy = reactor.getEnergyStored()
  fuel = reactor.getFuelAmount()
  maxfuel = reactor.getFuelAmountMax()
  waste = reactor.getWasteAmount()
  reactivity = reactor.getFuelReactivity()
  rate = reactor.getEnergyProducedLastTick()

  -- Begin Display --
  term.clear()
  term.setCursorPos(1,1)
  Print("Reactor Active: "..active..)
  Print("Stored Energy: "..energy.." RF")
  Print("Fuel Reactivity"..reactivity.." percent")
  Print("Total Fuel: "..fuel.." mB")
  Print("Total Waste: "..waste.." mB")
  Print("Total Contents: "..fuel+waste.." mB of"..maxfuel.." mB")
  Print("Energy Per Tick: "..rate.." RF last tick")
end

function Choice3()
  term.clear()
  term.setCursorPos(1,1)
  statistics()
  sleep(1)
end

function Exit()
  print("Shutting down. Goodbye.")
  os.sleep(2)
  os.shutdown()
end

mainMenu = {
[1] = { text = "Turn Reactor On", handler = Choice1 },
[2] = { text = "Turn Reactor Off", handler = Choice2 },
[3] = { text = "Detailed Status", handler = Choice3 },
[4] = { text = "Shut Down", handler = Exit }
}

function printMenu( menu )
 for i=1,#menu do
  if i == selectedItem then
   print(">> "..menu[i].text)
  else
   print("   "..menu[i].text)
  end
 end
end

function onKeyPressed( key, menu )
 if key == keys.enter then
  onItemSelected(menu)
 elseif key == keys.up then
  if selectedItem > 1 then
   selectedItem = selectedItem - 1
  end
 elseif key == keys.down then
  if selectedItem < #menu then
   selectedItem = selectedItem + 1
  end
 end
end

function onItemSelected( menu )
 menu[selectedItem].handler()
end

function main()
 while onMainMenu do
  term.clear()
  term.setCursorPos(1,1)
  print(colors.red, "Bardus Specifications Basic Underground Transcription Terminal\nProject Greenlight.\n")
  printMenu(mainMenu)
  event, key = os.pullEvent("key")
  onKeyPressed(key,mainMenu)
 end
end

-- End menu --

monitorOutput()
main()