You need to use chip and pin the first time you make a purchase with your new prepaid card in order to activate the contactless payment feature. Additionally, in line with the introducing of Strong Customer Authentication, you will only be able to make 5 contactless transactions in a row. The 6th transaction will require a PIN number and thereafter the next 5 transactions can be contactless.
This regulation is designed to protect you and your account. Without any limits on contactless payments, if you lost your card and a fraudster used it for contactless payments without any checks imposed on them, it would result in some substantial losses.
Unfortunately, the message you see at the card terminal on the 6th payment varies. It may appear that the transaction has been declined. You should try the transaction again but with the chip inserted and enter your PIN.