How to clear off the Error light from the Mitsubishi MELSEC PLC CPU unit?

Answer 1

Usually a PLC will reset upon power up and being placed in RUN mode. Try cycling the power and make sure that the PLC is in RUN mode.

After that you will have to check software and see if trouble code(s) are being set by the program.

Also check that all communications are working between the PLC and any modules and/or networked Slave units.