A little more info is needed. Since uploading to YouTube is not problematic, and many here have succeeded using standard settings, something else would appear to be going on.
What was the original video recorded using? Maybe a smartphone, in which case listing the model might help.
What is the original video resolution and frames per second; Is it 1920x1080 at 30fps, 1280x720 @30fps, or something else?
What is the video mode of your project inside shotcut?