error "cannot evaluate expression"

Hi, there is an error information when I solve two 1D-PDE models in comsol. How can I solve it? Thanks a lot.

By the way, I don't select any material, since the value of the solutions mean density distribution.

Cannot evaluate expression.
Failed to evaluate variable.
- Variable: mod2.H
- Geometry: 1
- Domain: 1
Failed to evaluate expression.
- Expression: mod2.H
- Plot: lngr1 (Line graph)