I don't know if you have played idle, but the cannons and mortars are represented by the same images used for them in idle. They are added as structures to the map, territories can have a cannon or mortar (they will not be removed nor can they be moved to another territory).
In your case, as far as the image tells me you don't own a cannon or mortar. This is due to the mod configuration of your game. It is either configured to place only a few cannons and / or mortars, or none.
If you do own a cannon or mortar, it can be the case that your target is not in range of them. The range is also configurable for both cannons and mortars.
You're not the first one asking these questions, I've had a small conversation lately about similar problems. I've decided to revamp / remake the mod, so you can expect either a second version or an updated Artillery Strike mod at some date in time!
Artillery Strike mod doesn't work?: 11/11/2022 23:23:44