After selling a stock, you have to wait until your money reaches your account to either reinvest it or withdraw it - what is often referred to as “settlement”.
Settlement is the time that stocks or cash take to move from one place to another. The regular settlement time for stocks or cash is the trade date plus two business days, commonly referred to as T+2. Customer payment for purchases in cash accounts must be made promptly, which typically means by no later than two business days following regular-way settlement (i.e., S+2).
Following a sale, your funds need to “settle” before you can withdraw them to your bank account. Regular-way settlement is the trade date plus two trading days (T+2) so on the third day, those funds will go into your buying power, and, assuming your withdrawal holding period has passed, funds will appear as withdrawable cash. This is why sometimes the amount available for withdrawal is different from Buying Power