Please double-check that the terminal you are paying at supports contactless payments. It’s also worth checking that contactless payments have been enabled within your security settings.
If you're paying in the European Economic Area and haven't used chip & PIN in a while, it's possible that you have exceeded the contactless payment limits in PSD2. Either reset your limit in-app, or make a chip & PIN transaction and you'll be able to continue using contactless.
If you have never made an in-store payment or if you have just received your new Revolut card, you need to make a chip payment before contactless becomes available.
If your physical card has been damaged or has worn out, you can order a replacement card. The estimated delivery date will be provided as soon as you have ordered a new card.