Unable to grade without submission

Clicking OK to start grading without a submission does nothing.

Workaround

Go to the assignment (Blackboard course -> Control Panel -> Course Tools -> Turnitin -> Assignment name)
Right click on the Assignment Inbox tab
Choose Open Link in new tab/window. (Or similar, depending on your web browser)

You will then be able to click on the pencil icon next to the student you wish to grade without submission.

With thanks to colleagues at the University of Leeds for highlighting this, and providing a workaround.