In STAAD.Pro moving loads are applied on beam elements only. The moving load generator simply ignores the existence of the plates and searches for beams in the vicinity of a wheel load and distributes the load on the beams as simply supported end reactions.
If your model consists of just plate elements, define dummy beam elements at the locations where the moving load is supposed to be applied. Ensure that these dummy members are connected to the plate nodes. For proper connectivity, you need to ensure that the beam elements are divided into same number of divisions as used for the plate meshing. Use dummy material for these beams having zero density and very low E value so that these do not contribute towards any load or structural stiffness. Once the model is analyzed, STAAD would load up these beams with the generated moving loads and these loads in turn are going to be passed on to the plates.
A sample model is attached for your reference Culvert_Vehicle_Load.STD