Get Posts via Taxonomy Term Name with Space

You could get the posts by slug instead of name. Like this:

$posts = get_posts(array(
    'post_type' => 'coupon',
    'numberposts' => -1,
    'post_status' => array('publish', 'unreliable', 'draft'),
    'tax_query' => array(
        array(
            'taxonomy' => 'stores',
            'field'    => 'slug',
            'terms'    => array( 'new-store' ),
            'include_children' => false
        )
    )
));

Or you could get the posts by term id. like this:

$posts = get_posts(array(
    'post_type' => 'coupon',
    'numberposts' => -1,
    'post_status' => array('publish', 'unreliable', 'draft'),
    'tax_query' => array(
        array(
            'taxonomy' => 'stores',
            'field' => 'term_id',
            'terms' => array( 4 ),
            'include_children' => false
        )
    )
));

Here you can find a guide on how to know the term id:
https://facetwp.com/how-to-find-a-wordpress-terms-id/