If you want to exchange money from selling shares, you must wait until the money is settled.
For example, you sell $1,000 worth of shares on the US stock market. Due to the T+1 settlement, you may not exchange your proceeds until one business day later, by which time your proceeds will have been settled.
To check the amount of money available for exchange, please go to the Accounts tab > Tap a Specific Account > More > Exchange > Amount Available.