Candy Rack App works perfectly on the pages built with PageFly, you would only have to check that you are using the "Product details" element. Product Details element is like a home that contains all necessary Shopify elements to ensure they're connected and synced along when one changes.

Please, make sure that the ATC button, as well as other elements needed for Candy Rack to do the job (Product variants, Quantity buttons, etc.), are inside of the Product Details element. When the ATC button is placed separately from the Product detail block, the Product Variant cannot be detected and thus a Candy Rack pop-up will not be displayed.