I can get the corner treatment dialogue box to show by picking an individual part, selecting Edit Properties and then picking an end point, but the change corner treatment options are grayed out. All the online examples show the Corner Treatment dialogue box in the Properties area just under Groups.
Steve;
I'm not 100% sure if this suggestion will fix your issue, but I just ran into something [at least] similar in SW2015 weldments.
I wanted to put an endcap on a weldment, but inside the tube (recessed), showing the appropriate chamfers that a welder would machine onto the pads before inserting them. The system showed the "corner treatment" box grayed out, and wouldn't allow me to click the little checkbox to open it up for change [between radius and chamfer].
I had chosen the face first, then the command, which my VAR tech support explained as a known issue.
Once I chose the command first, then the face, it allowed me to click the checkbox and change the corner treatment conditions.
STR #827722, for anyone that wants to look it up.
I don't recall if this annoyance still exists in SW2016