input: def add_matrix(matrix1, matrix2):
      """ Add two matrices element-wise. If matrices dimensions don't match, return an error.
      
      >>> add_matrix([[1, 1], [1, 1]], [[2, 2], [2, 2]])
      [[3, 3], [3, 3]]
      >>> add_matrix([[1, 1], [1, 1]], [[2], [2]])
      'Matrices not compatible'
      """
      if len(matrix1) != len(matrix2):
          return 'Matrices not compatible'
      if len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      rows = len(matrix1)
      cols = len(matrix1[0])
      result = [[matrix1[r][c] + matrix2[r][c] for c in range(cols)] for r in range(rows)]
      
      return result
output:     rows = len(matrix1)
      cols = len(matrix1[0])
      result = [[matrix1[r][c] + matrix2[r][c] for c in range(cols)] for r in range(rows)]
      
      return result