First go to a service centre and clean the charging port if that didn't work you need to replace the charging port if your device is in under warranty you will get a free replacement if it's not a physical damage
1 check the cable by replacing with other cable and try changing charger too see if that's work if not shutdown phone then plug the charger see if 2 lightning symbol appear if it did ur fast charging working if not try cleaning charging port still not working disable the fast charging and restart the device and again enable fast charging
In case the issue still occurs. Please register your concern in Samsung Members Application (Open Samsung Members Application > Support> Tap on error report > Type your query > Send).
