G12.1 is an option, is yours turned on?
The tool holder needs to be a "face milling" holder i.e. the endmill is in-line with the stock, not perpendicular or "cross milling" holder, do you have one of these?
Does your machine have Y-axis "up top"?
If you have the option, the correct holder and do not have Y-axis than G12.1 is what you need.
Answer these, and I will assist you further.