undefined pressure variable error

Hi!

I am new to comsol and I created this toy model to play with in order to get a better understanding.
The model intends to simulate the rizing air in a cylinder. The cylinder has an inner and outer radius. Between these two radiuses the cylinder is made of steel held at a fixed high temperature. The air is supposed to rize from the bottom where it is room temperature. I have modeled gravity in the z direction using a Volume Force. Running the model I get the following error

Undefined variable.
- Variable: pA
- Geometry: geom1
- Domain: 2
Failed to evaluate variable.
- Variable: comp1.mat1.def.rho
- Defined as: ((unit_kg_cf*comp1.mat1.def.rho((pA/unit_Pa_cf),(comp1.T/unit_K_cf)))/(unit_m_cf^3))

So it does not seem to recognize the pressure variable which the density of air depends on. The model is attached.

Any help is greatly appreciated! Also if other obvious newbie design flaws are spotted :)
Thanks!