php - Disable WooCommerce Checkout if cart count is not divisible by a certain number

Solution:

The hookwoocommerce_check_cart_items needs an "error" notice to disable checkout (which was not the case). I have also simplified the code:

// Check cart items conditionally displaying an error notice and avoiding checkout
add_action( 'woocommerce_check_cart_items', 'check_cart_items_conditionally' );
function check_cart_items_conditionally() {
    $multiple_of = 6; // <= Here set the "multiple of" number

    if ( ( WC()->cart->get_cart_contents_count() % $multiple_of ) != 0 ) {
        wc_add_notice( sprintf( __('You need to buy in quantities of %s products', 'woocommerce'), $multiple_of ), 'error' );
    }
}
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Optional: To be sure that checkout is not accessible you can add the following that will redirect checkout to cart page if cart items total count is not a multiples of N:

// To be sure: redirect checkout to cart page if cart items count is not a multiples of N
add_action( 'template_redirect', 'redirect_checkout_to_cart_conditionally' );
function redirect_checkout_to_cart_conditionally() {
    $multiple_of = 6; // <= Here set the "multiple of" number

    if ( is_checkout() && ( WC()->cart->get_cart_contents_count() % $multiple_of ) != 0 ) {
        wp_redirect( wc_get_cart_url() );
        exit();
    }
}
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Code goes in functions.php file of the active child theme (or active theme). Tested and works.