Why can't I trade the SPX, and how can I make a trade based on the SPX's movement?

The SPX is not tradeable, as it's note a stock, or and ETF, or an option, or a future, etc.  It's a mathematical result based on the prices of the stocks in the S&P 500, with more weight given to the stocks with greater market capitalization.  

However, you can trade the movement of the SPX in a number of ways.  

Options on the SPX are tradeable, and there are a number of ETFs that mimic the movement of the SPX, such as the SPY, which attempts to follow the SPX at 1/10th of the SPX price.  It's not perfect, but it's often close enough.  Another way to trade the SPX is to trade the Futures symbol ES, which is the SPX futures.  And there are other ways to trade the SPX's movement.

So while we can't trade the SPX itself, there are several ways to trade the SPX movement even if you cannot trade the SPX itself.  You can see these types of trades discussed and traded in our live trading rooms. Feel free to ask in the live trading rooms if you have more questions about how to place SPX-related trades.