Work through these checks:
Your card is Active. Check your card status in the app. A card that's still Pending can't be used.
Your card is in Apple Pay or Google Wallet. If you replaced your card or reinstalled the app, you may need to add it again.
Your card isn't locked. If it is, tap Unlock on your card in the app.
Your phone supports NFC and NFC is turned on. On Android, make sure Google Wallet is your default payment app.
The terminal accepts contactless payments. Hold your phone near the reader until it confirms.
You have enough balance to cover the purchase and the fee, and your balance isn't negative.
The purchase is within your limits: $1 to $2,000 per purchase, and up to $10,000 in any 24 hours.
If the payment was declined, open the transaction in your activity to see the reason. See Why was my card payment declined? for what each reason means.