Let me explain, if the user inputs "13" the value of amount wont be a number, it will be a string, example:

User inputs "18" Value of "amount": "18" (not a number)

So, instead, you should do this

io.write("enter a number  ")
amount = io.read()

isNumber = tonumber(amount)
if not isNumber then 
  print("no amount selected")
end