This error indicates Marlin is configured for use with one of the Arduino Mega-based 3D printer control boards that use an ATmega2560 microcontroller. The "Anet V1.0" board definition is for 3D printer control board based on the ATmega1284P microcontroller, and so the compiler detects that it is compiling for an incompatible board.
So this makes me think that Marlin is not configured correctly for the Anet A8. I would focus your attention on how you applied the A8 configuration to the Marlin code. If you will provide a link to the instructions you are following for this process. that might supply useful information to the helpers here.