How do I fix the user notification, "Ditch inlet type should be paired with a trapezoidal shaped gutter"?
This user notification is generated because by definition of HEC-22 a ditch inlet is trapezoidal in shape. This can be see in the help document titled, "Flow in Ditch or Median Section on Grade". Below is a screen shot of the page. What the user notification is telling you is that you need to match the gutter shape with the geometry of the inlet.
Make sure the shape of the gutter approaching the inlet (catchbasin) is of the trapezoidal shape. This can be set in the catchbasin properties. Please see the screen shot below.
The spread and depth are calculated just on the upstream side of the inlet. In this case, since the inlet opening will have some width associated with it, a trapezoid is always formed and hence the gutter shape defined at the catchbasin location must be set to Trapezoidal, to model the cross section at that location.
If you have a 'v'-shaped ditch that widens where the inlet is, then you can model the connected gutter link element(s) with the trapezoidal shape, using a near-zero bottom width value.