To setup Apple Pay Cash you need to have iOS 11.2 or later installed on your device, and you must setup the two-factor authentication for Apple ID. You can then setup Apple Pay Cash in the Wallet app. Here's how:
Apple Pay Cash
Apple Pay Cash allows you to make quick payments to your friends and family via the Messages app. Payments are deducted from your attached debit or credit card. All debit transactions are free. There is a 3% charge for credit card transactions.