How do I remobe black spaces on the sides in video?

Hello,

After exporting my Project the finished mp4 Product has black spaces on the sides. It does not look like that when i produce it in premiere pro, then I would have known how to correct it. However after exporting it it has black sides. Why is this? How can I get the video to fill the whole screen? I have tried all the options i "source scaling" when exporting it and nothing helps.

Maria