An error message is generated if the HC12 is sent a message it doesnt understand, ie cant parse.
Sounds like the MEGA is sending unwanted chars to the HC12.
Can you send data to the HC12 from another source, ie something like a PC serial terminal?
The other possibility is that you have changed the serial data format from the default (8 data bits , 1 stop bit) to something else.
ie the data rate is right, but the data bits and or stop bits are wrong.