Open the order and tap “Send cart”. If the 24-hour window is closed, the approved template goes out; if it is open, the message with buttons.
When the customer has no phone on file, the cart cannot be sent; add the number in Customers or send her the cart link on whatever channel you talk. The link shows her the cart and lets her pick delivery and payment without an account.
If it still does not work: check in Customers that the number is in international format (+44…). A number with a leading 0 is not valid for WhatsApp.