Flow Simulation Error: Not possible to calculate the internal task, if there are mass/volume flow openings but no pressure boundary conditions defined.

Hello,

I've got some problems with flow simulating my dual fuel cell setup.

No problems when checking the geometry and giving the inlet tube a boundery condition.

But when I press the run button and start calculating, I got the following error:

"It is not possible to calculate the internal task, if there are mass/volume flow openings but no pressure boundary conditions defined"

I'll hope someone can help me solving this problem.

SolidworksFlow Simulation