Eigenfrequency Error "Cannot evaluate expression"

I am trying to compute the Eigenfrequency physics for a mesh I created and imported into COMSOL. The mesh is 3D, complex, and asymmetrical but I have had no problems computing with it in other studies. However, when I run the study I receive the following error:

"Cannot evaluate expression.
Failed to evaluate variable.
-Variable: comp1.acpr.p_t
-Geometry: 1
-Boundary: 1
Failed to evalulate expression.
-Expression: comp1.acpr.p_t
-Plot: surf1 (Surface)"

I suspect the error has to do with the fact I cannot select my mesh as the Sound Hard Boundary (Wall) 1. It says (not applicable) next to the mesh selection. Can I somehow create a boundary of the mesh to use for this? Is this why I am receiving the above error?

Thank you