If you get the "Insufficient Funds" error when attempting to close a margin position, it can mean one of two things:
(1) You did not chose a level of leverage. See instructions for Closing Positions.
(2) Open orders are tying up funds which prevent some other orders from being placed, which may prevent orders to close positions from being placed.
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: