How to set a minimum order amount for Woocommerce excluding shipping and VAT?

In WooCommerce, I am using the following code to set a mandatory minimum order amount for checkout:

/**
 * 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 = 15;
    $cart_total = WC()->cart->total; // Cart total incl. shipping
    $shipping_total = WC()->cart->get_shipping_total();  // Cost of shipping
    if ( ($cart_total - $shipping_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 excl shipping 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 excl shipping to place your order' , 
                    wc_price( WC()->cart->total ), 
                    wc_price( $minimum )
                ), 'error' 
            );

        }
    }
}

But I would like to use instead the cart subtotal excluding VAT and shipping. Actually I am getting the cart total including VAT and shipping.

How can I get the cart subtotal excluding VAT and shipping?

You need to get the cart subtotal excluding vat, using WC_Cart get_subtotal() method instead of using the "total" property.

Also, your code can be simplified a bit.

Try the following replacement:

/**
 * Minimum order subtotal amount for checkout (excl shipping and taxes)
 */
add_action( 'woocommerce_checkout_process', 'wc_minimum_order_amount' );
add_action( 'woocommerce_before_cart' , 'wc_minimum_order_amount' );
 
function wc_minimum_order_amount() {
    $minimum  = 15; // <=== Here specify the minimum order amount
    $subtotal = WC()->cart->get_subtotal(); // Cart subtotal excl. shipping and taxes

    if ( $subtotal < $minimum ) {
        $message = sprintf( 
            esc_html__('Your current order subtotal is %s. To place your order, you must have a minimum amount of %s (excl. shipping and VAT).', 'woocommerce'), 
            wc_price( $subtotal ), 
            wc_price( $minimum )
        );

        if( is_cart() ) {
            wc_print_notice( $message, 'error' );
        } else {
            wc_add_notice( $message, 'error' );
        }
    }
}

It should work as expected.