Error: Cannot Use Accessory
"Cannot Use Accessory" or "Apple USB Ethernet Adapter: This accessory requires too much power"
The power adapter is not powerful enough. Use a better power source.
Not to worry! This is a common error and it means your iPad cannot get enough electric power for your attached accessory. The little Ethernet adapter thing is considered an accessory.
To remediate this you just have to be sure the iPad is charging, as indicated by a green icon in the upper right corner on iOS 12.
If it's not, be sure the power adapter is plugged in and that power is being supplied by a sufficiently powerful power adapter. The power adapter that comes with your iPad will always be sufficiently powerful to charge and power accessories.
The iPad Mini 4 for example comes with a 12W power supply like the one in the image below. You must use a power adapter that supplies this much power, or more power. Any drop in power will create problems.