WooCommerce dont allow Subscription and Regular product on same checkout.

functions.php

<?php

use WC_Subscriptions_Cart;

add_filter('woocommerce_add_to_cart_validation', function(){
  
  //If WooCommerce Subscriptions is installed handle the things...
  if (in_array('woocommerce-subscriptions/woocommerce-subscriptions.php', apply_filters('active_plugins', get_option('active_plugins')), true)) {
    
    $cartContent = WC()->cart->get_cart();
    
    if (is_array($cartContent) && ! empty($cartContent)) {

      $cartContainsSubscription = WC_Subscriptions_Cart::cart_contains_subscription();
      /** @var WC_Product|null|false $productToBeAdded */
      $productToBeAdded     = wc_get_product($product_id);
      $productToBeAddedType = $productToBeAdded->get_type();

      if (($cartContainsSubscription === true && $productToBeAddedType !== 'variable-subscription') || ($cartContainsSubscription === false && $productToBeAddedType === 'variable-subscription')) {
          
        $passed = false;
        wc_add_notice(__('You can not have "Subscription Product" and "Regular Product" in your cart at the same time.'), 'error');

      }
      
    }
  
  }

  return $passed;

}, 10, 2);

I once need to stop visitors to add Regular products and Subscription products at the same time on WooCommerce cart. And here is the solution.
In my case I had to prevent it for Variable Subscription products, thats why the product type on line 19 is set to variable-subscription, but you can extend the if condition to other options even if it isn't related to Subscription products...

You can add the above code on your theme functions or call it with your plugin if you are building one.