I am getting this error in my code saying Invalid use of void expression

In the function vector in_to_post(vector infix) (the vector=(1 + 2) * 3 ^ 4 ) is being converted to an output vector which is of the form (Postfix: 1 2 + 3 4 ^ *) and i am getting an error saying invalid use of void expression . I have commented the lines where i am getting this error.

    char get_optr(Token t) {
        // your code here
        return t.optr;
    }


vector<Token> in_to_post(vector<Token> infix) 
{
    vector<Token> output;
    // your code here
    stack<Token>b;
    for(int i=0;i<infix.size();i++)
    {
        if(is_opnd(infix[i]))
        {
             output.push_back (infix[i]);
        }
        else if(get_optr(infix[i])=='(')
        {
           b.push(infix[i]);
        }
       else if(get_optr(infix[i])==')')
        {
          while(b.empty()==false)
          {
              //char i=get_optr(b.pop());
            if(get_optr(b.pop())=='(')   // I am getting error here
            {
                break;
            }
            else
            {
               output.push_back(b.pop());  //error
            }
          }
        }
        else if(is_opnd(infix[i])==false)
        {
           // char i=infix[i];
            //char j=b.top();
            if((priority(infix[i])>priority(b.top()))||get_optr(infix[i])=='('||get_optr(infix[i])==')')
            {
                b.push(infix[i]);
            }
        }
        
    }
    while(b.empty()==false)
    {
        output.push_back(b.pop()); //error
    }

    return output;
}