Ask the customer to describe what happens.
If the charger LED turns red when the customer connects the vehicle, the cable is not properly connected or faulted.
If the charger LED turns red when the customer starts the charge (in the app or with RFID chip) there is no payment method added in the app or the RFID chip is blocked/invalid.
If the charge spot is publicly available, and the problem persists despite the above mentioned causes, you can try rebooting the charger in Connect and ask the customer to try again after 4-5 minutes - to eliminate connectivity issues.