Card payments: Is this the first time you are using your card? You will need to use your card at least once with a chip & PIN machine before you can use it contactless, this is to prevent abuse (in case of it being stolen during delivery for example).
Keyring payments: Please make sure that your device has been activated with the 9-digit code and that it is not frozen. You can check it directly in the app.
Please also keep in mind your local contactless payment limits. For example, in the UK the spending limit for contactless payments has been increased from £30 to £45. In Europe, it differs depending on the country and for the higher amount purchases you need to provide your PIN which you can find within your app.