Hi Everyone, 

Happy social distancing.
I'm attempting to use the new Flexible Component features in 2020 and am having a devil of a time getting it to work in the case of a simple Wave Spring

I'm trying to define the spring so that it adjusts to the height of the compononent it loads against.  
I've created a JIG assembly which contains the spring, and a block for it to load against.  Moving the block up and down adjusts the spring

There is a single external relationship between the wave spring sketch and the block

Based on the online documentation, I should be able to make this component flexible in another assembly by specifying a planar face as a substitute for the bottom of the block
But when I try to make the component flexible, I can't pick the face.  A tooltip says it's "not compatible.with the original entitity."

I've tried a couple of variations of this set up can't under any circumstances seem to get Solidworks to go along with my plan.
I've packed the assembly and jig models, including all of the part files into a zip file and attached them to this post.  
If someone would offer me some insight into this issue, I'd sure be appreciative.
Thanks
John