Firing (and ground targets in general) is simple but not necessarily intuitive.

Interacting with the cannons (hit X, possibly a few times) brings up the firing hotbar. If it's not appearing, try pressing R1 - you may have switched off the setting that brings it up automatically.

Use the "fire" button like a normal hotbar skill with R1+Circle. This brings up the target marker.

You may assume at this point that you hit Circle again to fire, but that just cancels your use of the skill. You need to use X to confirm your shot.