how to make author to write comment on only his own posts?

I have the same thing running on my website. Here is how I do do it…

Only post author and commentor can view each others comments

function restrict_comments( $comments , $post_id ){ 
global $post;
$user = wp_get_current_user();
if($post->post_author == $user->ID){
        return $comments;
}
foreach($comments as $comment){
    if(  $comment->user_id == $user->ID || $post->post_author == $comment->user_id  ){
        if($post->post_author == $comment->user_id){
            if($comment->comment_parent > 0){
                $parent_comm = get_comment( $comment->comment_parent );
                if( $parent_comm->user_id == $user->ID ){
                    $new_comments_array[] = $comment;       
                }
            }else{
                    $new_comments_array[] = $comment;   
            }
        }else{
            $new_comments_array[] = $comment;           
        }
    }
}
 return $new_comments_array; }

add_filter( 'comments_array' , 'restrict_comments' , 10, 2 );

Only allow post author to reply to a comment on their post

add_action( 'pre_comment_on_post', 'wpq_pre_commenting' );

function wpq_pre_commenting( $pid ) {

    $parent_id = filter_input( INPUT_POST, 'comment_parent', FILTER_SANITIZE_NUMBER_INT );

    $post         = get_post( $pid );

    $cuid         = get_current_user_id();

    if( ! is_null( $post ) && $post->post_author == $cuid && 0 == $parent_id ) {

        wp_die( 'Sorry, you can only "Reply" to a message - click on the Reply link to send a message to the member who messaged you' );

    }

} 

I also used a function to add a body class if the user was the current post’s author so I could style the comment form so that the post author could only see the reply comment field…

add_filter(
    'body_class',
    function( $classes ) {
        if ( is_single() ) {
            $post = get_queried_object();
            $user = wp_get_current_user();

            if ( $user->ID == $post->post_author ) {
                $classes[] = 'post-author';
            }
        }

        return $classes;
    }
);

css

.post-author #respond.comment-respond {display:none;}
.post-author .byuser #respond.comment-respond {display:inline;}