不能卖空，在金融市场上，通常是指投资者无法借入另一方的资金或证券来赚取现金。具体来说，当市场下跌时，卖空者会通过卖出证券的方式获得资金，随后再以更低的价格买回这些证券并最终将它们归还给借款方，从中获利。然而，如果市场不允许卖空，投资者就不能采取这种策略来赚取收益。

不能卖空的原因在于维护市场稳定和防止过度投机。如果允许卖空，当市场下跌时，大量卖空操作会导致股价进一步下跌，从而引发恐慌性抛售和市场的崩盘。此外，卖空操作也可能导致一些机构或个人利用卖空机制来操纵市场价格，损害市场的公平性和公正性。

因此，为了维护市场的稳定和保护投资者的利益，许多市场都不允许卖空操作。但是，这也会限制投资者的收益机会和风险控制手段。在允许卖空的市场中，投资者可以通过借入另一方的资金或证券来赚取现金，从而扩大自己的投资组合和风险控制能力。

英文对比阅读：

"Cannot short sell" means that investors cannot borrow funds or securities from another party to make cash. Specifically, when the market falls, short sellers sell the securities to get cash, and then buy them back at a lower price and return them to the lender to profit. However, if the market does not allow short selling, investors cannot use this strategy to make a profit.

The reason for not allowing short selling is to maintain market stability and prevent excessive speculation. If short selling is allowed, when the market falls, a large number of short selling operations will cause the stock price to fall further, leading to panic selling and market collapse. In addition, short selling may also lead some institutions or individuals to use the short selling mechanism to manipulate market prices, damaging the fairness and justice of the market.

Therefore, in order to maintain market stability and protect investors' interests, many markets do not allow short selling operations. However, this also limits investors' income opportunities and risk control methods. In markets that allow short selling, investors can borrow funds or securities from another party to make cash, thereby expanding their investment portfolios and risk control capabilities.