@missingno1 Hey, do you mind sharing a link to the video so we can see what you mean?
Although, If I had to take a guess, it’s because the resolution of your video is not widescreen native. Which means that it’s width and height ratio is not compliant with the 16:9 ratio that youtube uses.
If you exported with the default size that comes with Pencil2D (should be something like 800 by 600) you exported with a w:h ratio of 4:3, so youtube naturally has to compensate by surrounding the video with black bars to fit their widescreen native player. Old tvs had this ratio so is something normal in video standards.
If you require to change the size of your final video, double click on the camera and change the size to something like 960 x 540. If you already animated something and you increase the camera size too much it might be too obvious.
You should also be able to change the export size when you get to the options dialog before you render the file. But it would be better to work on the export size from the get go.