Ovviously you have not cleared all Pandoran bases within range of your base.
How many hours are given for the attack warning? How far are you in the game? Depending on whether you have done the Pandoran Citadel research you can try scanning for the Pandoran Base. Otherwise, you have to successfully defend a nearby haven, which reveal the offending base. If you have progressed far enough into the game, you should have enough troops and aircraft to cover missions and base defense.
You might want to read through some of these threads:
Base defense (Bug or feature)?
Cannot find any Pandoran bases