Fallback image URL if no featured image exists via a shortcode function

First, you have a typo in the provided code: an extra space between the apostrophe and the forward slash before “white-placeholder.”

Second, without examining the underlying logic in detail, I might try for the else statement:

else {

$placeholder_image = get_stylesheet_directory_uri() . '/white-placeholder-200x100.png';

return $placeholder_image;

}