If you want to export a video at 1080x1920, it starts with setting the correct Video Mode. That would be step #1.
Setting → Video Mode → Non-Broadcast → Vertical HD
When exporting, stay out of Advanced. Export will fully adhere to the Video Mode set.
Another example would be 1080x1080 with a 1:1 aspect ratio.
If your source video is wide, and you want to make it narrow, you’ll need to decide what to cut off the ends or have black letterboxing at the top and bottom. That would be done using one of the Crop filters applied to the clip(s) needed.
Additional resource to help better visualize how Resolution and Aspect ratios work. This link is set to 1080x1920.
Thanks for reply, but pronlems:
1.The original video starts in 1920x720 for the full version.
2. I make the shorts/reels from the full video.
IS there a better way to do this?
Applying any filter to force the height beyond 720, going to 1920 will result in distorted video. You would have to decide what portion of the original video would be visible.
To edit a different resolution to a Vertical Video Mode, you will need the filter Size, Position & Rotate. I did try Crop: Source with Center selected, but it left thin letterboxing at the top and bottom.
You could limit the size to 720 with Size, Position & Rotate for no distortion. Then decide which of the left & right would be visible.