An iPad might not charge if the connector is dirty or the USB adapter is not properly plugged into a power outlet. If it still does not charge after cleaning the connector and checking the adapter, the owner may try restarting it while it is connected to a power source.
When an error message such as, “This accessory is not supported by this device," is displayed, the USB adapter is the problem. The first step is to ensure that it is an Apple-certified adapter. If not, one should exchange it for an adapter that is and contact Apple Support if the device still does not charge. There could possibly be a hardware issue that needs more technical assistance.