Someone wrote 20 puts on SPY at 296 at some unknown time in the past. They collected a premium for this action. Because the strike price is well above the current price of the S&P 500, a buyer decided to exercise a few days early (American stock options can be exercised until expiration).
That exercise means they sold 2,000 shares of SPY at $296/share to the option writer. When this exercise occurred, Robinhood gives him overnight to add enough funds to his margin account to bring the account back in line with margin rules.
If that doesn’t happen they’ll sell the SPY, ASAP to reduce their risk of a really large loss. His actual loss will depend on the price Robinhood gets when they sell the 2,000 shares of SPY and the option premium he got for writing the options. At the moment it’s probably around $15-16k less the premium, but that could change tomorrow, depending on market prices.