Why do I get a thick grey color border after I publish my file in a video format?

I'm so sorry you've come across this problem, Mohammed. We've identified an issue in Storyline 360 where publishing to video produces a border in the MP4 output file.

I know how frustrating this can be for a course creator, as the goal is to produce high-quality work -- both visually and contextually. While I don't have any workarounds to share at this time, you're in the right place. We'll let you know as much as we can, as soon as we can!