The most common reason is that this public page does not have an assigned Trading Condition Setting. A public page must be linked to a Trading Condition Setting for visitors to place orders or submit inquiries. When not linked, the public page list and detail page will display in red text: "Trading Condition Setting not assigned. Visitors cannot place orders" with a "Go to assign" link. In V5, when creating a new public page, Trading Condition Setting is already required.
Another reason is that the Trading Condition Setting has no payment method configured. The checkout page will display "This page has no payment method configured. Orders are temporarily unavailable. Please contact the seller." Go to Seller Management → Trading Condition Setting, open the condition set, and select a payment method under "PO Condition Setting". If you want to use PayPal or PAYUNi, your company must first set up the corresponding payment gateway in Company Settings → Integration and Payment Gateway.