VAST error 400, aka General linear error, is an error that can be caused when a video player is unable to display the linear ad. It may occur when the Media File is not a valid video file of the specified format, or because the browser restricted auto play with sound, or another unknown reason. To fix the error, the publisher should check for player issues, enable the player to detect for restricted auto play and the buyer should ensure that the VAST response returns a valid creative.