Shorts/Reel "cut off" on left and right sides?

[Hudson555x]

You need to start by choosing the correct Video Mode for your project.

Settings > Video Mode

Introduction Video Mode is the project resolution, aspect ratio, frame rate, and color space. It is important to set this as you desire at the beginning of your project. If you try to change it later Shotcut will try to retain the timing of edits, but some users have reported problems, and it is risky. Also, at this time Shotcut will not automatically adjust anything for you related to size and positioning of elements like text and picture-in-picture when changing resolution. You can think of …

At the bottom of this post, you can find many other forum posts, where others have learned about the Video Mode

Changing the Video Mode after making multiple edits is risky, and Shotcut warns of this.

I can only guess that you went into Export → Advanced and changed settings.
There is a warning before you click on Advanced.

Knowing what Video Mode you need before you start is vitally crucial, and often misunderstood. Once the Video Mode is set, there is no need to enter into Export → Advanced unless you know what you are doing.

Guessing from your very first post, a vertical Video Mode is needed.
Most likely, you’ll need to start a new project from that vertical Video Mode.