You may get a message about “not enough shares" if you already have an outstanding pending order for the shares you want to sell with a new order.

If you get this message, you’ll need to cancel any outstanding orders before you can sell the shares.

To cancel a pending order in the app:

1. Select Account (person icon) → Menu (3 bars) or Settings (gear)
2. Select History
3. In Pending, select the order to cancel
4. Select Cancel Order

For more details, check out Cancel a pending order and How to buy stock.