You are getting a pivot error with the universal joint because the center beam has no connection.
This is the reason for the extra body. You need a body to connect a universal joint from each beam coming into the corner, so there would be three universal joints on the top corner body and three universal joints on the bottom corner body.
If you drop a Modal analysis onto the Model line of this project, then in Mechanical, drag and drop the Fixed Support onto the Modal system you can solve and see all the beams that are loose.
If share topology is used, then the beam ends get welded together, then this becomes a weldment and beams will experience bending loads. A true truss carries only tension and compression and no bending moments are put into the links.