If you get the "Insufficient Funds" error when attempting to close a margin position, it can mean one of two things:
(1) You forgot to chose a level of leverage.
Solution: See instructions for Closing Positions.
(2) You have open orders that are tying up funds.
Open order may prevent more orders from being placed.
Solution: Cancel all your open orders (even stop orders), and try again.
Your open orders can be found by navigating to the “Trade” > “Orders” tab from the menu and you can cancel them by clicking on the red “X” to the right of the order: