How to set a post featured image as background image in WordPress

I created a section where I display all my post content. My struggle is now to set the post thumbnail as a background image.

Here the structure of the section:

<section id="testimonials"> 
    <div class="testimonials-container">
        <div class="heading-container">
            <h3 class="page-title">Testimonials</h3>
        <div class="testimonials-box">
                $queryposts = array(            
                    'post__in' => array(75,73), 
                    'post_type' => 'post',
                    'posts_per_page' => -1,  
                    'order' => 'ASC' 
                $lastblog = new WP_Query( $queryposts );            
                if($lastblog->have_posts() ):
                    while($lastblog->have_posts()): $lastblog->the_post(); ?>

                    <div class="testimonial-wrapper">
                        <div class="testimonial-item">   
                            <p class="test-content"><?php the_content(); ?></p>
                            <p class="test-title"><?php the_title(); ?></p>
                            <?php $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' );?>
                            <div class"image-class" style="background-image: url('<?php echo $thumb['0'];?>')"></div>
                    <?php endwhile;

Here my css:

section#testimonials {
    height: 400px;
    background-image: url(images/testimonials-bg.jpg);
    background-size: cover;
    background-repeat: no-repeat;
    text-align: center;


.testimonial-wrapper {
    width: 100%;
    max-width: 900px;
    margin: 0 auto;

    display: block;
    width: 125px;
    height: 125px;
    background-size: cover;
    background-repeat: no-repeat;

When I try to view the page the image does not show and tying to inspect the page it shows this:

background-image: url((unknown));

How can I set the thumbnail as background image of my div box?

Hope you can help

Read more here: How to set a post featured image as background image in WordPress

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