ER 420M Sam4s till Error message?

This is normal. Your machine is closed until you open it. To open it for trading, simply enter your clerk secret (PIN) number followed by the clerk button. If you have forgotten your code, there is a programming manual here:

http://the-checkout-tech.com/manuals/cashregisters/Samsung_ER-420M_operating_programming_manual.html

Registerman
December 2013