Your order will be executed during the US stock market hours, during which the stock price may change. If you place an order for 1 share of stock and the price is $100 at the time, and you have exactly $100 left in your account. When the market opens, the stock may have risen to $102 per share. Since your account balance is insufficient, your order will be cancelled. If it's not this situation, please contact customer service for further inquiries.