When you open your positions, keep in mind your total used margin must not exceed your free margin.
Here is an example.
Let's assume your account balance is €10,000, with €3,333 being already in use.
Thus, your free margin is €6,667.
Now you want to invest €700 in EUR/USD with 500 multiplier. The margin you will need to open this trade is:
Margin = 700*500*1/30 = €11,667
Thus, your free margin, €6.667, is less than the margin requirement for this position, €11,667; this is why your trade won't be placed and you will see the Insufficient Funds warning on your screen.
In order to resolve this issue, you have to either invest a smaller amount or set a smaller multiplier.
If you want to trade with no leverage restrictions, you can apply for the Professional Client status. This is what you have to do.