Why is there no order button on the menu page?

The menu page is for browsing; the Order tab is the one door into ordering, showing only the ways you have switched on.

Because the menu tab is for browsing only — there are no Add buttons on it by design, so a customer can read the menu without being pushed into an order. The Order tab in your customer pages is the one way in: tapping it opens a small chooser with up to three buttons — Order to your table, Order for collection and Order for delivery — showing only the ways you have switched on (with just one way on, there is no chooser and the customer goes straight in). The same chooser opens from the Order card on your venue page and from the one-line Ready to order? pointer on the menu page, and a customer who scans a table QR code skips it entirely.

If you cannot see the Order tab either, check three things on Orders → Settings: the Order & Pay master switch is on, at least one way of ordering is on (at your table, collection or delivery), and Show the Order tab and card on your venue page has not been switched off — it sits beside Flexible table entry whenever ordering is on, and also under Collection & delivery while collection or delivery is on. The full walkthrough is in How do I set up and run Order & Pay? under "How customers start an order".