When you are calling your lease function, within your print, you are not passing the parameter that will be used. There is also an error in your return, your indentation is wrong, so the second return is inside the if, and will never be executed.
arrend(numero)
In this case, I think you want to pass the variable num2 as a parameter. It would look something like that, fixing the return too.
def arrend(numero):
if num2 - math.floor(num2) < 0.5:
numero = math.floor(num2)
return numero
return math.ceil(num2)
num2 = float(input("digite um número não inteiro: "))
print("o número inteiro correspondente é: {}".format(arrend(num2)))