setting mass flow rate at inlet

Hi,
Model description

I have a 3D laminar air flow inside channel whose cross section is rectangle. for BC ,
1- the inlet , mass flow rate boundary is set (kg/s),
2- the outlet , zero pressure p=0 pa
3- the all other boundaries are symmetry.

My problem:

When I run it, I have a problem with convergence , then eventually failed to find the solution

Thanks