Before a shot can be rendered as video, it needs an approved storyboard still.
This is enforced in the pipeline, not just suggested in the UI. Attempting to generate video for an unapproved shot is refused with an explanation.
The reason is economic: a video generation costs roughly ten times what a still does. Approving composition, framing and blocking at still prices — and rejecting the ones that are wrong — is the single largest cost saving available to you.
In Beginner mode the gate still runs; storyboards are generated and auto-approved on your behalf. In Director and Professional modes you approve them yourself.