Meaning of "Segregated solver steps do not involve all components"

Hi,

I am making a simple model of a microwave waveguide, something I have done before, but I cannot get the solution to compute because of this error (attached jpg). Usually when I get COMSOL errors, it references some problem in the solver, however the solution turns out to be an error in how I set up the physics of the model. However, I am running out of things to try and I really wish I could trace the error back to the source in a more orderly fashion.

I'm attaching the model as well for reference.

Thanks.