Apple Pay Later (APL) service is currently discontinued. There are references to it in some support material and probably in older software. Apple is turning instead to current BNPL services. They require the device be on iOS 18.x.x and if they accept Apple Cash is up to their terms and conditions.
Afterpay is currently available and several other services are expected too. The apple support article I shared the link to below goes into more detail and explains other options available.
https://www.affirm.com/wallet/applepay
Pay with installments using Apple Pay - Apple Support