input: from typing import List
  
  
  def add_matrix(matrix1: List[List[int]], matrix2: List[List[int]]) -> List[List[int]]:
      """ Adds two matrices and returns the resultant matrix. If the matrices are not compatible for addition, it returns 'Matrices not compatible'.
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])
      [[6, 8], [10, 12]]
      >>> add_matrix([[1, 2], [3, 4]], [[1, 2, 3], [4, 5, 6]])
      'Matrices not compatible'
      >>> add_matrix([[1, 2, 3], [4, 5, 6]], [[1, 2, 3], [4, 5, 6]])
      [[2, 4, 6], [8, 10, 12]]
      """
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