input: Write a function to count down from a given positive integer to 1 using a while loop and print each number. The function should take a single positive integer as its input and return None. If the input is not a positive integer, the function should print an error message.
  assert count_down(6)==None
  assert count_down(3)==None
  assert count_down(-5)
output: def count_down(n):
      if n <= 0:
          print("Error: Input must be a positive integer.")
          return
      while n > 0:
          print(n)
          n -= 1