Solving heat equation, error:6194 "Attempt to evaluate non-integral power of negative number"

I am trying to solve heat equation with a coupled system, Poisson's equation, convection and diffusion and heat equation.

When I try to solve the heat equation to obtain N I am getting this error, Could some one please help me.

I am attaching the screenshots of solver manager, error message, constants, and scalar expressions.

Thank you very much