Slitherine

How does one move an AA gun so it is under the enemy aircraft? It keeps prompting me to attack the aircraft.

There was somewhere/somewhen a recommendation to carefully move the cursor around the edge of the target hexagon, I can't remember by whom.
That didn't work reliably for me. I was able to do it in one occasion in the US Pacific Pearl Harbor scenario when I had an AA gun on the road (which allowed four tiles of movement) and the JP aircraft was three (road) tiles away i.e. outside the normal range of the gun. My hypothesis is that the collision box of the combat prediction is too large.
On a separate occasion during the Burma Road campaign I was able to use the multiple-turn movement order in a way that the AA gun stopped under the JP aircraft in the first turn. I fully admit that that is a finicky workaround which won't work in every situation.
Hope this helps.

RIGHT-CLICK and not LEFT-CLICK on the hexagon you want to go on