In this weldment, the long-ways diagonal braces do not join the rest of the frame.
At the top end the joints show as beam ends, while at the bottom it appears that they have bonded perfectly.
I can't identify any difference in the joints so why would the ends be treated differently?
In any case, running the Twist Test shows the two arrowed joints coming apart with an obvious, bad, effect on the stress sharing in the frame.
To make matters worse, I cannot even attempt to create a connection set which specifically bonds the members together. Right-clicking on Connections has no effect!
It would be great if someone would step-by-step me through the correction process, but failing that a link to a video, or a tutorial, or a textbook which covers this problem would be much appreciated. This sort of thing seems to be a huge mystery, but someone who reads this forum must have solved the problem, and I need your help.