If you define a field to be not empty on a transition (transition enforced), and then perform the transition without entering a value the error message thrown to the user is not meaningful and it does not give any clue to the user as to what he needs to do so that the error is not thrown. It should function in a similar way as the current Scrum cardwall, when a transition is done on the scrum cardwall and if there is a transition validation rule, the modal is displayed to the user with a meaningful message so that the user can enter a value for the field described in the error message (please see screen capture)