WMS 3.0 :- While picking, error received "Some shipments are without location. We are trying generating again. Please try after some time."

Sometimes, user get the above-mentioned error while picking in WMS 3 as shown below :

The above error while assigning picker is usually shown when some selected shipments don't have order item location generated against the items while assigning the picker.
Once the order item location gets generated and when the picker is assigned again, the order will move to assigned.