Implementation of PDEs with two dependent variables

Hello,

I have to model a fixed bed reactor step by step regarding species transport, heat transfer and flow for my bachelor thesis and first I want to study the flow by using the Brinkman Equations. For this, I'd like to use the mathematical interface because the equations in the flow interface aren't exactely what I'm looking for and aside from that I got an error using this interface "Undefinierter Wert gefunden.
- Detail: Undefinierter Wert in gefunden.Der Gleichungs-Restvektor.
Es gibt 40501 Freiheitsgrade liefern NaN/Inf im Vektor für die Variable comp1.u.
bei Koordinaten: (0.03,0), (0.0299864,0), (0.0299728,0), (0.0299579,0), (0.029943,0), ...
Es gibt 40501 Freiheitsgrade liefern NaN/Inf im Vektor für die Variable comp1.w.
bei Koordinaten: (0.03,0), (0.0299864,0), (0.0299728,0), (0.0299579,0), (0.029943,0), ...
- Feature: Stationärer Löser 1 (sol1/s1)" and I absolutely don't know what the problem could be.
I have no experience in modelling with Comsol Multiphysics and I don't know how to implement the necessary PDEs, especially because there are two dependent variables with two different units in the Brinkman equation (pressure and velocity).
I would be so grateful, if you could help me with this problem and maybe also with the error message described above.

Kind regards,
Tiziana