Volume fading is an essential tool in music production - it could be creating the perfect intro to your rock song or an awesome build-up to your Dubstep drop - and of course it's possible in Soundtrap too.
First, make sure you're in the Editor view and that you can see the region you want to fade. Then click and drag the white fade anchors in the uppermost corners of the region (left for fade-ins, right for fade-outs) towards the middle of the region (1). You'll see a white line indicating the duration of the fade (2).
Another way to create fades is to hover above the region and click "Edit", then choose "Fade in" or "Fade out". If you're using the Soundtrap app, click a region and choose Fade to activate the fade anchors, then deselect the region to deactivate them, or click the region again and choose "Trim".
(1) Fade anchors
(2) White overlay indicating fade duration