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Hills, mountains and woods block line of sight (LOS).
Thus if there is a highland or forest hex between the attacker and its target - the attack is impossible.
If yoiu select a unit:
Read more about hex highlights.
A hero's Battle Armor (to the lower-right) cannot shoot at the Self-propelled Mine (upper-left) because the hills block the line of sight.
But it can attack the Drone. The Drone hovers over the highlands and not behind them, so it can be shot at.
Tip: if you've moved your unit but it came out that you cannot attack the desired target - just undo your last move with Cancel button.
Some units have an Indirect Fire ability that allows shooting over obstacles. Artillery is a good example.
Trace: