Velocity render element cannot be combined with motion blur effect. The render element will not be shown until motion blur is disabled.

This message appears if motion blur is enabled while attempting to render velocity render element. Both geometry and camera motion blur must be disabled to render this element. This is because the velocity element is used to add motion blur in post-processing. Adding 3d motion blur to the element would result in incorrect, doubled motion blur in the final result.

Solutions:

• Disable both geometry and camera motion blur in Render Setup > Camera> > Motion blur, then render the element
  
  OR
  
  
• Remove the velocity element and render scene with motion blur enabled