When Amazon says “Temporarily out of stock. Order now and we’ll deliver when available,” they mean the item is currently not in stock but still available for ordering. They tell you that once they receive feedback from their supplier, they will email you with an expected delivery date. View availability estimates, here.
If an item is temporarily out of stock you can still add it to your cart. Amazon should notify you once it is restocked.
Visit the product detail page and choose item/s to order. “Add to Cart.” When you're done, click the shopping cart icon. If you need to edit; save for later or delete next to the item. Proceed to checkout and follow the instructions. Review your information. Place your order to buy.
How long the item is unavailable varies and depends on the supplier, demand, manufacturing capacity, supply chain logistics, etc.
You can contact Amazon customer service for any stock or delivery queries or issues, here.