I had this warning message today that didn't make much sense and a google search provided zero hits, so I thought I'd make this discussion for the next person who encounter this misleading message. I was attempting to make a broken-out section in my top view so I could dimension the depth of my counter bored slot. It was my intention to make the section at a depth of 15 mm so it would be through the center of my feature. At 14 mm deep everything works fine (the preview shown is at 14 mm) but at 15 mm I received this message "Cannot delete CVs from the poorly constrained sketch." The sketch used to create my broken-out section was an unconstrained spline so I thought maybe there was something funny where the start and end joined together. Deleting and re-drawing the sketch led to the same error.
To make a long story short the problem is from the cutting plane of the broken-out section being tangent to the pre-drilled hole for my M6 tap in the right hand view. At 17.5 mm from the edge with a 5mm pre-drill, the math puts the tangent at 15 mm down. When I moved the M6 hole, the broken-out section worked. When I changed the M6 hole to a 5 mm drilled hole I received the same error so it's not related to taps or cosmetic threads. I haven't done any additional testing but that should fix most people's problems. When you see this message, check for tangent edges in other views.
The questions are: What is a CVs and to what poorly constrained sketch is it referring?