I'm currently trying to simulate a vacuum environment on a 3" x 7" panel with a lattice structure inside and am running into some confusion. I don't necessarily need a fixture because there's an equal 14.7 psi force on each side of the rectangular panel, but I don't think SOLIDWORKS is allowing me to proceed without a fixture. How can I proceed to test this panel? The red is the pressure and the green was a displacement fixture on the face (which I think is wrong).

Thanks!!