These are the most reported issues for an iPhone 6 with faulty charging. You may try some home fixes that may resolve the issues

• Change the wall adapter or the port the phone is being charged in if it doesn’t charge at all.
• Check if there is no debris or dirt in both the charging dock and the charging brick.
• Make sure the USB is connected firmly on both ends.
• Check the charging brick or the lightning cable for visible signs of damage. Example: Bent pin, loose cable, etc
• Make sure you have no damage in your charging dock if unsupported device warning shows.