The issue comes from the active theme. You should contact the theme support. The issue cannot be fixed by wpDiscuz side. 

just contact the plugin support and ask them if they use the function. 

As I've already mentioned you should make sure the <?php comments_template() ?> function exists.

function comment_support_for_my_custom_post_type() {
     add_post_type_support( 'my_post_type', 'comments' );
}
add_action( 'init', 'comment_support_for_my_custom_post_type' );

and in the wp-includes/comment-template.php does the code excist:

function comments_template( $file = '/comments.php', $separate_comments = false ) {
global $wp_query, $withcomments, $post, $wpdb, $id, $comment, $user_login, $user_ID, $user_identity, $overridden_cpage;

if ( ! ( is_single() || is_page() || $withcomments ) || empty( $post ) ) {
return;
}

if ( empty( $file ) ) {
$file = '/comments.php';
}

still missing the form...

Probably the plugin that creates a single page don't use <?php comments_template() ?> function. Check out the last two points in this doc:

https://wpdiscuz.com/docs/wpdiscuz-documentation/getting-started/missing-comment-form/#tip_5

I tried to use the plugin for a single page but the form ist still missing. Have a look at the screenshots. Do i miss something?

You'll need to call the shortcode with post_id, in the above-mentioned code, and check again. It should work fine.

For example:

Please note: post_id the id of the current Post, and the post_id is deferent in each post.

It does not work... Reverted - shortcode has no access to the rating function - loads but only placeholder image.