I think your indentation needs to be adjusted? Like so:

  def collatz(n):
      while n != 1:
          print(n)
          if n % 2 == 0:
              n = n // 2 
          else:
              n = n * 3 + 1

      print(1)

Otherwise, n = 1 terminates, and n != 1 gets stuck looping at lines 2-3.