Why can't I buy stocks right away after I've just sold them, and say I don't have enough capital?!!

Moomoo Buddy : Dear customer, US stock trading involves T+0 trading, which means you can buy and sell repeatedly on the same day. T+2 settlement means that the settlement of money and goods requires two settlement dates. For example, stocks sold on T day need T+2 days before funds can be withdrawn. According to your description, it is recommended that you go directly to My - Online Customer Service - Manual Customer Service or call the hotline (+65) - 6321 8888. Thank you for your understanding and I wish you a happy life!