Error - Failed to evaluate variable jacobian

Hello,

I am currently working with the batteries and fuel cells module and I am attempting to create a model of a micro-scale PEM fuel cell. Right now I am starting by just trying to solve a free and porous media flow physics to begin with, but I am getting an error that says:
"Failed to evaluate variable Jacobian
-Variable : U_in_Anode
-Geometry: 1
-Boundary: 5"

Block 5 is the anode chamber of the cell, and U_in_Anode is the volumetric flow rate of the anolyte. What is likely to be the error in this, and how can a fix it?

Thank you in advance.