You do not get a specified velocity boundary when you use a mass inflow defined by an STL surface. The MFIX solver internally converts these types of “boundary conditions” to point sources. This is why the GUI only allows you to specify the mass flow rate, just like a point source. You can provide an optional velocity to create a momentum source, however, this value is not imposed like a specified velocity boundary condition.
That said, your mesh is far too coarse for both fluid and PIC calculations. Ideally, you should have at least three full (uncut) cells across the flow width. Most of your flow domain contains no full cells.