input: def add_matrix(matrix1: List[List[int]], matrix2: List[List[int]]) -> Union[List[List[int]], str]:
      """
      Adds two matrices if they have the same dimensions. Returns 'Matrices not compatible'
      if the matrices cannot be added.
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])
      [[6, 8], [10, 12]]
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6]])
      'Matrices not compatible'
      """
output:     if len(matrix1) != len(matrix2) or any(len(row1) != len(row2) for row1, row2 in zip(matrix1, matrix2)):
          return 'Matrices not compatible'
      
      result = []
      for row1, row2 in zip(matrix1, matrix2):
          result.append([a + b for a, b in zip(row1, row2)])
      
      return result