The S&P 500 must stay above 6,400 for the rally to continue — after August jobs report rattles market, strategist says

1. S&P 500 critical level at 6,400 must hold for bulls. 2. August jobs data showed only 22,000 jobs added, raising concerns. 3. Unemployment rate increased to 4.3%, indicating economic slack. 4. Investors wary of rate cuts and labor market issues impacting sentiment. 5. September historically is weak for stock returns, affecting outlook.