I test my code not working I checked file name and verified phone number for custom thumbanail.

function myFunction() {
   function setThumbnail() 
{
  YouTube.Thumbnails.set(
    videoId= "DfqGVS0_w14",
    media_body= DriveApp.getFilesByName('ddp.jpg').next().getThumbnail()
  ); 
}
}