get_post_meta shortcode returns empty field

The shortcode callback function should always echo its output, not return it. Your code should be something like this:

function wcal_abandoned_cart_id_shortcode_callback( $atts ) {
    $atts = shortcode_atts( array(
        'post_id' => get_the_ID(),
    ), $atts, 'wcal_abandoned_cart_id' );

    echo get_post_meta( $atts['post_id'], 'wcal_abandoned_cart_id', true );
}
add_shortcode( 'wcal_abandoned_cart_id', 'wcal_abandoned_cart_id_shortcode_callback' );

If the output of your callback was HTML, besides echoing, you could just write your HTML outside of the PHP tag.