Check three things in order:
1. Is the mode set to "Display Only"? In Display Only mode, there is no shopping cart and all "Add to Cart" buttons are disabled. 2. Is the item out of stock? When the Trading Condition Setting has "Show inventory quantity" enabled, items with 0 stock display "Out of Stock". If the Trading Condition Setting does not have "Allow orders when out of stock" enabled, the item cannot be added to cart. 3. When prices are hidden, does the catalog allow inquiries? For items with hidden prices, the item can only be added to cart if the catalog has "Allow buyer inquiries" enabled, or the Trading Condition Setting has "Accept custom requests" enabled.
Also note: The "Add to Cart" button on a product card adds 1 unit at a time. The "Add to Cart" button next to each price tier in the product detail adds items according to that tier's minimum order quantity, and it accumulates in the cart rather than replacing items.