Welcome to Android Central!  Can you try a different S6 quick charger?  Maybe bring it to your local Best Buy Samsung Experience store and ask to test out one of their quick chargers.  If that charger works fine, then your charger is probably faulty--contact Samsung to see if they can replace it under warranty.  If other chargers do the same thing, then there may be a problem with the charging port.  That would require a trip to a repair shop (or sending it back to Samsung if it's under warranty).