The reason why the “Modify” button becomes inactive when you are trying to set Stop Loss or Take Profit levels for an already open position is that you may have placed these levels too close to the current price on a Fixed account and on a Standard account with Instant Execution. It can also mean that you have set the levels incorrectly in relation to the current price.
Please note that for short positions (sell orders), Stop Loss levels must be set above the market price and Take Profit levels – below the current market price.