Cannot get or show publish thumbnail in wordpress

i am engaged on a WordPress plugin and i am attempting to show/get the publish thumbnail under the publish on my “Associated posts” half. I attempted many issues that are purported to work however not for me … right here is the code (on my plugin.php file):

add_theme_support( 'post-thumbnails', array( 'publish', 'web page' ) );

if ( has_post_thumbnail() ) {
    echo ('there's a thumbnail');
    echo get_the_post_thumbnail(get_the_ID(), "thumbnail"); // nothing
    echo get_the_post_thumbnail(); // nothing
    echo var_dump(get_the_post_thumbnail(get_the_ID(), "thumbnail")); //string '' (size=0)
    echo var_dump(get_the_post_thumbnail()); //string '' (size=0)
    the_post_thumbnail('thumbnail'); // nothing
    the_post_thumbnail(); // nothing
}
else {
    echo ('there is no such thing as a thumbnail');
}

first i assumed that there have been no thumbnail for my publish however my first echo returned “there’s a thumbnail” and all the remaining does not return something … so i do not perceive… what’s lacking ?

Thanks

Edit:

right here is the present full code in my plugin.php file:

operate Similar_Article ($content material) {
    if (is_single() || is_page()) {
        the_post();
        $similar_to_add = '';
        $id = get_the_ID();
        $content_and_similar = $content material . '<br><br>'; //areas between publish and associated posts

        /* many strategies referred to as to search out my associated posts */

        if (has_post_thumbnail()){
            $content_and_similar .= get_the_post_thumbnail(); // it really works
        }
        else {
            $imageData = wp_get_attachment_image_src(get_post_thumbnail_id ( $id ), 'thumbnail'); 
            $content_and_similar .= '<img src=" '.$imageData[0].' ">';
        }

        $content_and_similar .= $similar_to_add;
        return $content_and_similar;
    }
    else {
        return $content material;
    }
}

add_filter( 'the_content', 'Similar_Article' );

my operate works if there’s a thumbnail however show nothing if there is no such thing as a thumbnail…