If an instrument's price which you wish to trade with has increased, the message comes up as a warning that you might purchase a higher value than your available funds allow you to. Its aim is to prevent you from going into a negative balance if the order is placed at a higher price.
With an entry equal to 100% of the account value, even the smallest market movements might make the trade unaffordable and reject the order.
In this case, you could try opening the order with less quantity of units below 95% of your available free funds.
NOTE: This rule does not apply to Value Orders, where you can use up to 100% of your free funds to place a single Value order.