VAST error 406, aka Missing required mezzanine file, is an error that can be caused by a VAST response when returned without a required mezzanine file. Publishers with server-side ad implementations often require mezzanine files, which are raw high-quality video files for ad-stitching, or to generate video files of appropriate quality. To fix the error, the publisher should alert the buyer and the buyer should ensure that a mezzanine file is included in the response when required by the publisher.