New to the game - problem...

[Mongo]

You've encountered one problem and you want to give up on the game already?

The artillery pieces do not have spotters. They can only fire at what the artillery crew can see.

Left click on the unit. Then click on area fire and move your curser around. The arrow from the curser to the unit has to be orange for the unit to fire at the curser. If the arrow is black the line of sight is blocked.

Click on the unit and then hit enter. This will show you what the unit can see. Then click enter again to release your view from the unit.