I don’t think it’s the middle piece, but maybe? See below image. This time I selected every face, and it’s still yielding the same result.

I have tried selecting faces, vertices, edges not of them are extruding properly for this shape.
Once you watch the video can you tell me what he is selecting? It’s not really clear he just shortcuts into selecting everything except for the piece and in the video it’s very difficult to understand what took place.
Thanks for working with me,
Jonathan