Hi George,
since you need jump-terms, you set the skeleton=True flag. Such integrators calculate the element matrices for both elements at the facet at once.
This requires both finite elements, and both element transformations.
These FacetBilinearFormIntegrators don't provide the usual CalcElementMatrix, thus the exception.
The two-element CalcElementMatrix is not available from Python.
I recommend to use a hybrid-DG like formulation for the (normal-derivative) jump term, then you don't need the skeleton integration.
An example for C0-interior penalty methods for the plate equation is here:
ngsolve.org/docu/latest/i-tutorials/unit...der/fourthorder.html
Best,
Joachim