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).
Please also keep in mind your local contactless payment limits. For example, in the UK it is 30 GBP. In Europe, it is higher but you need to provide your PIN which you can find within your app.