Apple Pay
Y can’t I pay with Apple Pay at a store card reader when I have plenty of funds on my Apple card
What type of “Apple Pay” do you have?
AppleCard ?
Other registered Credit or Debit Card ?
AppleCash ?
Apple Gift Card ?
AppleID Balance ?
And have you configured ApplePay on your phone?
Set up Apple Pay - Apple Support
Pay particular attention to Supported Regions and Banks.