CAM Studio unknown post error

I've built a very simple job, with four holes in a rectangular plate.
I left almost all settings to default, on a 3-Axis Generic Milling GRBL.
Verifying is working well.
When simulating, I don't see the machine, as I saw in one tutorial, even if I'm choosing for example a Fanuc.
When trying to post the operation, I'm receiving an "unknown post error".
How van I find where am I wrong?