/**
 * Set a minimum order amount for checkout
 */
add_action( 'woocommerce_checkout_process', 'wc_minimum_order_amount' );
add_action( 'woocommerce_before_cart' , 'wc_minimum_order_amount' );
 
function wc_minimum_order_amount() {
    // Set this variable to specify a minimum order value
    $minimum = 18;

    if ( WC()->cart->total < $minimum ) {

        if( is_cart() ) {

            wc_print_notice( 
                sprintf( 'Your current order total is %s — you must have an order with a minimum of %s to place your order ' , 
                    wc_price( WC()->cart->total ), 
                    wc_price( $minimum )
                ), 'error' 
            );

        } else {

            wc_add_notice( 
                sprintf( 'Your current order total is %s — you must have an order with a minimum of %s to place your order' , 
                    wc_price( WC()->cart->total ), 
                    wc_price( $minimum )
                ), 'error' 
            );

        }
    }
}