How can I enable Trailing SL on my position in Options Scalper?

You can enable trailing stoploss on a position by two ways:

Bottom Trade Panel: By tapping on Add SL, you can trigger the SL order form. Therein, you can set the Trailing SL on by selecting in the order form

Option Chart: When you have an active SL, that is visible on the option chart. You can tap on the SL pill on chart, within the further setting on SL, you should be able to add Trailing SL by tapping on "Add Trail"