i have two types of products in my store , one is in stock products and the other one is on request product. On request product is set as out of stock . If a user needs an on request product he will will give a request , then from backend we will approve the request then in the oser account page we will give an proceed to checkout button which is an direct checkout . The issue is if i added the product to checkout page like this( Domainname/checkout/?add-to-cart=product id ) iam getting an error message like this(You cannot add “product 1” to the cart because the product is out of stock).

for checking the validation i have used the woocommerce_add_to_cart_validation hook but its only working if any of product is there in cart .