Allow checkout only when a product of a mandatory category is in cart

Solution:

Here you have a solution that will make the trick. There is especially 2 main functions (the last ones):

1. The first function (N°3) display your message on cart page, when there is something in cart but not the mandatory product category. Displays also the message on the mandatory product archive pages (useful when customer get redirected from checkout, see below).
2. The second function (N°4) redirect customer to the product mandatory category archive pages when it tries to checkout and his cart has not that missing mandatory product category.

Define before your mandatory category slug in your_mandatory_category_slug() function.

This is the code:

// Function that define the mandatory product category
 function your_mandatory_category_slug(){

     // DEFINE HERE the SLUG of the needed product category
    $category = 'clothing';
    return $category;
 }


// Conditional function that returns true if the mandatory product category is in cart
function has_mandatory_category(){
    $category_needed = your_mandatory_category_slug();
    $has_cat = false;

    // Iterrating each item in cart and detecting…
    foreach ( WC()->cart->get_cart() as $item ) {

        // Detects if the needed product category is in cart items
        if ( has_term($category_needed, 'product_cat', $item['product_id'] ) ) {
            $has_cat = true;
            break;
        }
    }
    return $has_cat;
 }


// Function that display a message if there is not in cart a mandatory product category
function mandatory_category_display_message() {
        $category_needed = your_mandatory_category_slug();

    // check that cart is not empty (for cart and product category archives)
    if( !WC()->cart->is_empty() && ( is_cart() || is_product_category( $category_needed ) ) ){
        $category_obj = get_term_by( 'slug', $category_needed, 'product_cat' );
        if ( is_wp_error( $category_obj ) ) return;

        // Display message when product category is not in cart items
        if ( !has_mandatory_category() ) {
            $category_name = $category_obj->name;
            $category_url = get_term_link( $category_needed, 'product_cat' );

            // render a notice to explain why checkout is blocked
            wc_add_notice( sprintf( __( '<strong>Reminder:</strong> You have to add in your cart, a product from "%1$s" category, to be allowed to check out. Please return <a href="%2$s"> here to "%1$s" product page</a>', 'your_theme_domain'), $category_name, $category_url ), 'error' );
        }
    }
}
add_action( 'woocommerce_before_main_content', 'mandatory_category_display_message', 30 ); // for product mandatory category archives pages
add_action( 'woocommerce_check_cart_items', 'mandatory_category_display_message' ); // for cat page


// Function that redirect from checkout to mandatory product category archives pages
function mandatory_category_checkout_redirect() {

    // If cart is not empty on checkout page
    if( !WC()->cart->is_empty() && is_checkout() ){
        $category_needed = your_mandatory_category_slug();

        // If missing product category => redirect to the products category page
        if ( !has_mandatory_category() )
            wp_redirect( get_term_link( $category_needed, 'product_cat' ) );
    }
}
add_action('template_redirect', 'mandatory_category_checkout_redirect');

This goes in function.php file of your active child theme (or theme) or also in any plugin file.

This code is tested and fully functional.