Your card comes partially activated, which means you can use it through Apple Pay and Google Pay, and shop online before you have the physical card. Before using the physical card you have to activate it.
That can be done in 2 ways:
Activate it on your home screen or go to the respective card in the app
Use the card in a store and input your PIN-code. You have to put the card in the payment terminal the first time
Your PIN-code is always available in your app. Simply find the respective card in your app, and tap "Show PIN"