$S&P 500 Index (.SPX.US)$  is a stock index that provides information on the performance of a basket of securities so you cannot trade SPX directly.
https://www.thebalance.com/trading-stock-indexes-1031061
https://www.investopedia.com/ask/answers/how-can-i-buy-sp-500-fund/
There are ETFs that track SPX like   $SPDR S&P 500 ETF (SPY.US)$   $iShares Core S&P 500 ETF (IVV.US)$   $Vanguard S&P 500 ETF (VOO.US)$   $SPDR Portfolio S&P 500 ETF (SPLG.US)$ $SPDR S&P500 US$ (S27.SG)$ and these...