You can only fire at active maps, that means either your own main colony, or any of your outposts that are being attacked. Once you select the target for the artillery strike, you can then specify where on the map you want the shells to land.

It is not possible to just blindly shell enemy positions, if that is what you intended.

Look for mods that load a cell into an active map if you want to bombard cells where nothing is going on at the moment. For example Save Our Ship 2 (which I think is still not updated to 1.4) has a scanner that you can point to any cell and load that map, so then it's active and you can do airstrikes on it. Possibly there are mods other than SOS2 that do that.