Is there a way I can fade out subtitles ?
This subtitle tool seems not to accept effects.
What exactly are you hoping to be able to do?
You cannot apply clip effects to subtitles, they are not clips, and they are rendered by the player when the video is played, using options configured by the person who is playing them, not when it is created.
That is the primary feature which sets them apart from ‘titles’. They give the viewer control over what captions they want to see and how they want to see them.
ASS does have a \fad tag for simple fade in/out and \fade for more complex manipulations, but it isn’t guaranteed that all players will support them.