Make sure Stocard is added to your Apple Wallet and is set as the default card. Learn how to do it here:
How do I set Stocard as the default card in Apple Wallet?
First, look for the contactless symbol 
at checkout everywhere Mastercard® is accepted.
If your iPhone has Face ID enabled, simply double-click the side button and glance at your iPhone to authenticate with Face ID, or enter your passcode. Hold the top of your iPhone within a few centimeters of the contactless reader until you see “Done” and a checkmark on the display.
If your iPhone uses Touch ID, simply rest your finger on the Touch ID. Hold the top of your iPhone within a few centimeters of the contactless reader until you see “Done” and a checkmark on the display.