Issues with no flux boundary conditions in NDSolve

enter image description hereThe no flux boundary conditions are making trouble. How to handle them? The problem I’m trying to solve is as follows: PDE: 0.036*D[C[x,y],x]==2.4*10^-10*(D[C[x,y],{x,2}]+D[C[x,y],{y,2}]) Boundary conditions: 1. C[0,y]==Piecewise[{{1,H/21)==0 3. (D[C[x,y],y]/.y->0)==0 4. (D[C[x,y],y]/.y->0.005)==0

I wrote the following: sol2=NDSolve[{0.036*D[C[x,y],x]==2.4*10^-10*(D[C[x,y],{x,2}]+D[C[x,y],{y,2}]),C[0,y]==Piecewise[{{1,H/21)==0,(D[C[x,y],y]/.y->0)==0,(D[C[x,y],y]/.y->0.005)==0},C,{x,0,1},{y,0,0.005}]

The error message shows-The dependent variable in the boundary condition Dirichlet condition need to be linear. But I noticed that a similar no flux boundary condition works just fine (for a different problem though). sol3=NDSolve[{D[u[t, x], t] == D[u[t, x], x, x],u[0, x] == 0,u[t, 0] == Sin[t],(D[u[t, x], x] /. x -> 0)== 0},u, {t, 0, 10}, {x, 0, 5}]

What is the issue in the previous case and how to resolve it?