If you tried to place an order to open a position using margin and it was cancelled with the reason "Insufficient margin", then this means that the exchange's margin pool for that currency is currently used up. You can see the reason for cancellation when you click on the ID of the cancelled order.
In the case of position opening limit orders, margin is not reserved when you place the limit order so if the margin pool is empty when the price of your limit order hits then you will find that the order cancels with the "Insufficient margin" reason.
We have a limited amount of funds allocated for margin trading. However you can try and place a new margin order right away after a previous order was cancelled.