If you get the "Insufficient Funds" error when you place an order, it could be due to the following:
You have open orders that are tying up your funds
Open orders may prevent more orders from being placed.
Solution: Cancel open orders (even stop orders) and try again.
Your open orders can be found by navigating to the Trade page, then viewing the Open Orders widget. You can cancel them by clicking on the “X” to the right of the order.
You do not have funds for the selected currency pair
Placing a spot order for a currency pair which you do not have funds in will result in this error. For example, trying to place a BTC/USD order when your account currently only holds CAD.
Solution: Select a currency pair which matches the currencies you currently hold.
You did not enable margin
If you were looking to place a spot position on margin but did not enable margin, you can get this error. When margin is available for a market, there will be a toggle in the top right corner of the Order Form widget.