The VideoLAN Forums

1) There 's an mp4 video
2) there's a subtitles file in .ass format

I tries to burn subtitles into the video (create so called hardsub) via the "Strem" function. It took some time to calculate the results but the resulting file was just a raw video file without subtitles.

How do I solve this issue?

What's way better at that is Handbrake. Find that, it's a safe community created video processor. https://handbrake.fr/downloads.php

There's lots of guides and help around. I think it's one of the best windows programs I've ever used.