Why is the Modify position button greyed out when I try to set SL/TP for an existing order?

This may be because you have placed your stop-loss or take-profit levels too close to the current price. alternatively, it may mean that you have set these levels on the wrong side of the current price.

Please note that for short positions (sell orders) stop-losses must be set higher than the current market price and take-profit levels must be set lower than the current market price.

You can view the minimum stop levels for each platform in the symbol's specifications: https://www.fxpro.com/trading/forex