How to resolve Index Error in my code?

[supuflounder]

That helps a lot. I also added statements to print the result on each return statement, and got

Output:
Enter total number of leaf nodes=8
Enter Leaf Value 1/8: 3
Enter Leaf Value 2/8: 5
Enter Leaf Value 3/8: 6
Enter Leaf Value 4/8: 9
Enter Leaf Value 5/8: 1
Enter Leaf Value 6/8: 2
Enter Leaf Value 7/8: 0
Enter Leaf Value 8/8: -1
minimax(depth= 0 , nodeindex= 0 ,  True ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  -1000 ,  1000 )
.minimax(depth= 1 , nodeindex= 0 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  -1000 ,  1000 )
..minimax(depth= 2 , nodeindex= 0 ,  True ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  -1000 ,  1000 )
...minimax(depth= 3 , nodeindex= 0 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  -1000 ,  1000 )
... => [ 0 ]  3
...minimax(depth= 3 , nodeindex= 1 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  3 ,  1000 )
... => [ 1 ]  5
.. =>  5
..minimax(depth= 2 , nodeindex= 1 ,  True ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  -1000 ,  5 )
...minimax(depth= 3 , nodeindex= 2 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  -1000 ,  5 )
... => [ 2 ]  6
.. =>  6
. =>  5
.minimax(depth= 1 , nodeindex= 1 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  5 ,  1000 )
..minimax(depth= 2 , nodeindex= 2 ,  True ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  5 ,  1000 )
...minimax(depth= 3 , nodeindex= 4 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  5 ,  1000 )
... => [ 4 ]  1
...minimax(depth= 3 , nodeindex= 5 ,  False ,  [3, 5, 6, 9, 1, 2, 0, -1] ,  5 ,  1000 )
... => [ 5 ]  2
.. =>  2
. =>  2
 =>  5
The optimal value is : 5

This does not correspond to your output, but since I made no real changes in the code, I'm putting the code here:

MAX, MIN = 1000, -1000
 
def indent(depth):
    for i in range(depth):
       print(".",sep="", end="")

# Returns optimal value for current player
# (Initially called for root and maximizer)
def minimax(depth, nodeIndex, maximizingPlayer,
            values, alpha, beta):

    indent(depth)
    print("minimax(depth=", depth, ", nodeindex=", nodeIndex, ", ", maximizingPlayer, ", ", values, ", ", alpha, ", ", beta, ")"
    )

    # Terminating condition. i.e
    # leaf node is reached
    if depth == 3:
        indent(depth)
        print(" => [", nodeIndex, "] ", values[nodeIndex])
        return values[nodeIndex]
 
    if maximizingPlayer:
 
        best = MIN
 
        # Recur for left and right children
        for i in range(0, 2):
 
            val = minimax(depth + 1, nodeIndex * 2 + i,
                          False, values, alpha, beta)
            best = max(best, val)
            alpha = max(alpha, best)
 
            # Alpha Beta Pruning
            if beta <= alpha:
                break
        indent(depth)
        print(" => ", best)
        return best
 
    else:
        best = MAX
 
        # Recur for left and
        # right children
        for i in range(0, 2):
 
            val = minimax(depth + 1, nodeIndex * 2 + i,True, values, alpha, beta)
            best = min(best, val)
            beta = min(beta, best)
 
            # Alpha Beta Pruning
            if beta <= alpha:
                break
        indent(depth)
        print(" => ", best)
        return best
 
    # Driver Code
 
 
if __name__ == "__main__":
    scr = []  # List for Leaf Nodes
    x = int(input("Enter total number of leaf nodes="))
    for i in range(0,x):
        y = int(input("Enter Leaf Value "+ str(i + 1) + "/" + str(x) + ": "))
        scr.append(y)
 
 
 
    print("The optimal value is :", minimax(0, 0, True, scr, MIN, MAX))

I do not understand why MIN and MAX are global variables, but I didn't try to debug the code. Looking at the output, perhaps you can figure out why it does not deliver the same result as you indicated that it should. I am very suspect of your use of MIN and MAX which you never change. But that's just a guess.

The key takeaway here is the use of print statements to tell you what is really happening.