What is VAST error 501

VAST error 501, aka Non-linear ad creative dimensions do not align with creative display area, is an error that can be caused when a non-linear ad is unable to play because the creative dimensions did not align with creative display area (the creative dimension was too large). It may occur if you use CSS styles to set width and height of the video content element. To fix the error, the publisher should ensure that the set width and height match the display area. If you set the width and height of the video content element with CSS styles, set them with HTML attributes instead.