add_action( 'woocommerce_checkout_process', 'wc_minimum_order_amount' );
add_action( 'woocommerce_before_cart' , 'wc_minimum_order_amount' );
add_action( 'woocommerce_before_checkout_form' , 'wc_minimum_order_amount' );

function wc_minimum_order_amount() {

    $minimum = 6;
    $cartqty = WC()->cart->get_cart_contents_count();

    $url = htmlspecialchars($_SERVER['HTTP_REFERER']);
  $backtext = "<a style='color: white;' id='cartbackbutton' href='$url'>Hi there, We have a minimum order quatity of 6 jars, please click here to pop back and choose $cartqty more jars of yumminess.</a>";

    if ( $cartqty < $minimum ) {

        if( is_cart() ) {
            echo ('<style>.wc-proceed-to-checkout {display:none;}</style>');
            wc_print_notice( $backtext , 'error');

        } else {
            echo ('<style>.wc-proceed-to-checkout {display:none;}</style>');
            wc_print_notice( $backtext , 'error');
        }
    }
    else {
        echo ('<style>.wc-proceed-to-checkout {display:block;}</style>');
    }
}