You will try this
<?php $url = wp_get_attachment_url(get_post_thumbnail_id($post->ID), 'full'); ?> // Here you can manage your image size like medium, thumbnail, or custom size
<img src="<?php echo $url ?>"
/>
Check the code below and let me know if it works for you.
<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
<div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">
</div>
<?php endif; ?>