Plan to Clean the Earth is a side mission in Stellar Blade that takes place in the Wasteland. You will receive this mission shortly after defeating Maelstrom and retrieving the second Hyper Cell. It is given via a distress signal that comes in through the Airborne Squad channel.

I received the distress signal while in the middle of a fight with a Naytiba somewhere in the Wasteland, so I'm not sure what exactly triggers it. I'm guessing it comes in after a certain amount of time has passed since you left Atless Levoire during the main story of Stellar Blade. Either way, the mission is automatically accepted and your first objective is to find the source of the distress signal.

To accomplish this goal, you don’t just need to find the source of the signal. You need to go there. Adam has already found the source, and tells you that the signal is coming from the Scrap Yard in the Wasteland. The Scrap Yard is not to be confused with the Junkyard, which is also in the Wasteland, but in a completely different location. The Scrap Yard is located all the way north, and is marked on your map as a strange pink smiley icon.

You can approach the Scrap Yard from either the east or west, but there’s a locked gate blocking the eastern path, so unless you’ve already unlocked it, approach from the west. To be honest, this route is less of a path and more of a series of short cliff drops, but if you’re unsure, keep checking the map to make sure you’re dropping off the correct cliff.