If you enter your card PIN incorrectly too many times when you're trying to make a purchase, it will get blocked until 23:59 on the same day.
From there, you'll need to head into your Coconut app to get your PIN. Tap "PIN" in the Card Drawer in the "Account" tab (you'll need your security number from your card).
Equipped with your PIN, head to a UK ATM that offers PIN Services and follow the instructions to unblock your PIN.
Unfortunately we can't help you unblock your PIN without heading to an ATM. As the PIN gets blocked on the debit card itself, the unblock process has to be done at an ATM because the card needs to be physically present.