error when adding a velocity field to v flow battery model

Hi ,
I am new to Comsol. Recently, I am trying to build a vanadium flow battery model. I found the v flow model in model libraries is very useful. The model is too simple and I wanna add more to the model. I tried to add a velocity field using laminar flow or brinkman equation. However, the error comes out when I compute the model, It says undefined value found......Can anyone help me?

Thanks,