I was going through the weldments tutorial and encountered this problem.
A couple of trims were to be performed earlier and I was successful.
I have to trim the upper left corner with the vertical member as bodies to be trimmed and boundary being the top face of the horizontal member.
When I try to perform it as guided by the tutorial, I don't get the preview and not able to build it.
The error shown is 'Bodies do not trim each other completely. Please check your input.'
I have attached the picture and the file. Please have a look and clarify the problem.