Currently, Zip is only available on items which are in stock with Mighty Ape, so if any of the items in your trolley are listed as 'Available from supplier' rather than 'In stock', Zip won't appear as a payment option in your trolley. To use Zip for your purchase, remove any items from your trolley that are not in stock, and purchase these items using an alternative payment method.
Alternatively, if you qualify for Zip Gift Cards, you can purchase a Mighty Ape e-gift card via the Zip app to spread the cost of this over time, then purchase your desired items using the gift card.
More details on how to shop with Zip at Mighty Ape are available here.