To find the Aguda Cliffs checkpoint key you may need to leave the checkpoint completely, go do something else for a while, then come back later and see if it has appeared. This is because the Aguda Cliffs checkpoint key appears to be missing because of a bug. The bug is making the entire checkpoint inactive for some players, so there are no soldiers defending it, you can’t capture it, and there’s no key to be found anywhere. And without the key, you can’t get into the checkpoint’s guard station (or so it seems).
If you arrive at the Aguda Cliffs checkpoint and it’s functioning normally (meaning there are guards to attack), then you should be able to find the key resting on top of a concrete barrier on the opposite side of the road from the guard station. But if the checkpoint isn’t functioning normally, then you’ll have to get creative.
Some players are reporting that if they go away and do other stuff for a while, the Aguda Cliffs checkpoint will eventually function normally. But if you don’t feel like doing that, there’s another way to get into the Aguda Cliffs checkpoint guard station. Just use any weapon that does blast damage on the door, a regular frag grenade for example. If you’re not sure, you can check the Arsenal tab on the pause screen to see what types of damage your various weapons do.
Inside the guard station you’ll find an FND cache and various other materials, as well debris from the explosion you just caused.