The checkout button is not on the page
Two causes cover almost every report.
The page is not published. The button is an element on that page. Saving in the builder does not put it on the public URL. Publish the page, then open the live URL.
The button is hidden on the device you are using. The element settings are Hide On Desktop, Hide On Tablet, and Hide On Mobile. A button that works on a computer and disappears on a phone has Hide On Mobile on. Turn that off, save, and publish again.
Also confirm a business exists and the product is not still pending. See Before you sell and Product pending approval.
Frequently asked questions​
It works on my computer and not on my phone.
Open the button and turn off Hide On Mobile. Publish again.