What does it mean when an "Invalid S/L or T/P" warning prevents me from placing an order?

When placing an order, you have the option to make certain specifications such as your stop loss and take profit. If these price levels are entered incorrectly the order will not be placed after clicking "Ok" and the prompt "Invalid S/L or T/P." will be displayed at the bottom of the dialog box. 

Here is an example of how the stop loss or take profit might be incorrect: if you were to place a buy market order at 1.45918 and then set your stop loss at a price higher than this price e.g., 1.45993 then this would be an invalid stop loss as it would really be behaving like a take profit--It wouldn't be stopping your loss, it would be stopping your profit. 

The best thing to do when you have this problem is to use the horizontal line chart object (found in the chart objects toolbar) to map out your stop loss and take profit before you place the order. The lines will display the price at which they are set and you can use those values to enter the prices in their appropriate places in the "Place Order" dialog box.