How to pass external variables to the wp_new_user_notification_email filter?

I believe the problem is you’re using get_stylesheet_directory_uri() function in an attempt to include a file. This function returns a URL when what you want to do when you need to include a file is provide a path. You could be using instead is get_stylesheet_directory()

Here’s what the output would look like:

// - https://yourwebsite.com/wp-content/themes/my-theme/
$url = get_stylesheet_directory_uri();

// - \www\httpdocs\wp-content\themes\my-theme\
$directory = get_stylesheet_directory();