Highlighted shell has zero thickness error.

Hello,

Can anyone explain the error "highlighted shell has zero or negative thickness"?  I understand what it means but I don't understand what it tells me because:

A. When I click on the error message nothing is highlighted

B. The run still solves though I'm doing a SPA

C. When I run AutoGEM mesher, my model meshes with no warnings.

The model is a frame that has mostly shelled sheet metal plates with a couple solid element tubes/rods and connections made via weld features.

Any help or insight would be appreciated.