When i run this program i get an error that MinimaxNode' object has no attribute 'value'
ConnectFour is an other class that initializes the board and marks the moves, checks if someone won, etc.
Utility just returns 2 points (it is still in progress)
The problem arises that in MinimaxPlayer in child.get_minimax_value as it puts an error that MinimaxNode' object has no attribute 'value'
Your output you see is simply because you print out the object MinmaxNode, which must not have a __repr__ method set up. 
Your error of "local variable 'col' referenced before assignment" is because you define col conditionally in your for loop but use it as an argument in play_turn regardless of whether it's been assigned.  You should check that col has been defined after running your for loop and before calling it.
Minimax For Connect four doesn't return a value