Since you are using a stack, and the session clearly talk about a stack, at the end of this solution you are doing a JOIN with a stack. A stack is a structure that only can be iterated with pop() method (you only know the last element), and join takes the first position of a interable, till the last position, breaking the concept of a stack. This solution follows the real concept of a stack

def removeDuplicates(self, s):
        # ToDo: Write Your Code Here.
        stack = []
        result = []
        for pos in range(len(s)):
            if stack and s[pos] == stack[-1]:
                stack.pop()
                continue
            stack.append(s[pos])
        
        while stack:
            result.append(stack.pop())
        
        result.reverse()
        return "".join(result)