When using the Cs point group, NWChem assume the mirror plane being perpendicular to the z-axis (i.e. xy plane).
Therefore, you need to rotate the molecule to satisfy this constraint as shown here
geometry
N 0.0000 0.0000 0.0000
H -0.9377 -0.3816 0.0000
H 0.4689 -0.3816 0.8121
H 0.4689 -0.3816 -0.8121
symmetry cs
end