how to enter a Channel thickness

Hi everybody,
how to enter the Channel thickness?
I use the (laminar flow -> mass flow -> mass flow rate -> normal mass flow rate), but get an error!
I read a comsol documentation and I think I must enter a Channel thickness (maybe!!), but don't find where I can to enter a Channel thickness!
please help me, 1- how to enter a Channel thickness? 2- why get an error when I choose "normal mass flow rate" ?

thanks a lot