Need help

--# add to the top of your main function
function main() --#you have this line, it's just here for reference
local input = read()

--#change last line to
while true do
   main()
   sleep(0.1)
end

function setOn()
  rs.setOutput("back", true)
end
function setOff()
  rs.setOutput("back", false)
end
function clear()
  term.clear()
  term.setCursorPos(1,1)
end
function printOn()
  print("Licht Status: An")
end
function printOff()
  print("Licht Status: Aus")
end
function main()
clear()
input = read()
  if input == "An" or input == "an" then
	clear()
	setOn()
	printOn()
	main()
  elseif input == "Aus" or input == "aus" then
	clear()
	setOff()
	printOff()
	main()
  else
	clear()
	print("Gib entweder ,,An'' oder ,,Aus'' ein!")
	os.sleep(3)
	main()
  end
end

• You only set the input variable once wit read() and then never change it again, so the user can't change it without restarting the program.
• You didn't define "An" or "Aus" as a string so the program looks up for the variables An and Aus which doesn't exist.
• You defined the functions as ,,setoff" and ,,seton" but then called the functions ,,setOff" and ,,setOn"

while <condition> do --# Replace <condition> with your conditional statement

end

--# Here's a while loop that runs forever:
while true do

end

function seton()
  rs.setOutput("back", true)
end

function setoff()
  rs.setOutput("back", false)
end

function clear()
  term.clear()
  term.setCursorPos(1,1)
end

function pos(x , y)
  term.setCursorPos(x,y)
end

function printOn()
  print("Licht Startus: An")
end

function printOff()
  print("Licht Startus: Aus")
end

while true do --# Runs forever
  clear()
  local input = read() --# Gets user input (local means only this program can access the variable)

  if input == "An" then --# Quotes ("") makes it a string, not quotes makes it a variable
	clear()
	setOn()
	printOn()
  elseif input == "Aus" then
	clear()
	setOff()
	printOff()
  end

end

local input = read() --#here
function seton()
  rs.setOutput("back", true)
end
function setoff()
  rs.setOutput("back", false)
end
function clear()
  term.clear()
  term.setCursorPos(1,1)
end
function pos(x , y)
  term.setCursorPos(x,y)
end
function printOn()
  print("Licht Startus: An")
end
function printOff()
  print("Licht Startus: Aus")
end
function main()
  if input=="An" then --#here
    clear()
    setOn()
    printOn()
    main()
  elseif input=="Aus" then --#here
    clear()
    setOff()
    printOff()
    main()
  end
end
clear()
printOff()
main()