input: 
  def add_matrix(matrix1, matrix2):
      """
      Function that adds two matrices. Matrices must have the same dimensions.
      If the matrices are not compatible due to different dimensions, it returns
      an error message.
      
      >>> 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]], [[1, 2], [3, 4]])
      [[2, 4], [6, 8]]
      """
      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[i][j] + matrix2[i][j] for j in range(cols)] for i in range(rows)]
      
      return result
  
output:     rows = len(matrix1)
      cols = len(matrix1[0])
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(cols)] for i in range(rows)]
      
      return result