You may not be able to buy/sell certain stocks when:

- The stock is under surveillance (ASM, GSM, SMS) by exchange
- There are no buyers/sellers for these stocks (low liquidity). In this case, you can still place a limit order, but market orders will be disabled.
- You’re trying to sell a trade-to-trade (T2T) stock before delivery. T2T stocks can be sold after delivery only, i.e., 2 working days after an order is placed (T+2 days).