The S6400 does not have an image size that corresponds to 19:16 aspect ratio.
The S6400 image mode/image size 4608x2592 corresponds to 16:9 and the rest of the image modes/image sizes corresponds to 4:3 aspect ratio. To resize your images to any other aspect ratio, you will need to do it in a picture editing software.
Having the camera Continuous Mode set to120fps, 60fps, or Multi Shot 16 will disable the image mode option Refer to page 71 of the manual for more information.
See page 10& 11 or page 49 (reference section) of the S6400 manual for information selecting an image size.