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)
2. Select History
3. In Pending, select the order to cancel
4. Select Cancel Order

To cancel an open order in Robinhood Legend:

1. Add a recent orders widget to your layout
2. Hover over a pending order
3. Click Cancel

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