Why does my Apple Pay transaction says pending

The receiver has not accepted the transaction yet. You have the opportunity to cancel it if you wish. Note the support article here:

 If the transaction status says pending, the receiver hasn't accepted your payment yet, and you can cancel it. 

If you have an issue with an Apple Cash transaction - Apple Support

What payment method was being used ? (e.g. AppleCash, credit card, debit card, …)

See: “Accept a payment in Wallet” here:

Send and receive money with Apple Cash - Apple Support

What kind of thing are you buying? It may require processing in different stages and it is waiting for the next one.