If you attempt to place a "number of shares" pending (Limit, Stop or Stop Limit) order worth more than 95% of your free funds, you will receive a pop-up message. This rule is in place to protect you from investing more funds than you have available.
To avoid this, try reducing the number of shares you wish to purchase. Alternatively, you can place a "value" type market order, which allows you to use all of your free funds.
Note: When placing a value order using all your funds, volatile market movements may still trigger a negative balance order.