Ok, so there are black areas to the sides in your preview. Those remain in the video because the “canvas” (=video mode) that you’re working on is a big horizontal rectangle.
Now I am guessing you want to cut the sides so the final video is just the vertical rectangle that we see now in the video, right?
For this, there is a better option than crop (so disable it for now), it is called Reframe. See here for details on how to use it. Basically you select an area on the screen that the final video will contain.