After you find Chen Zhen at Meteorite Crater, he will ask you to find some units of Iron, Copper, and Tin. For the most part this is pretty easy. Simply mine the crater's interior for large amounts of iron and small amounts of copper, as well as some other resources (like sulur, limestone, marble, etc.). Though you can find an abundance of iron here and some copper, however tin is very hard to find, but can be found one at a time near copper or limestone.
In the quest description you are told that tin only comes from "copper ore." At first you may think that you have found copper ore because you have previously found copper in the crater, but do not be fooled. Copper ore is a material that you can mine where the main component is copper and tin is the secondary component (like the iron ore where copper is the secondary component).
In order to find tin, you must find copper ore. To do this you must simply press "H" (for standard controls), click on the "Scan" icon at the top left of the Help window, click "Cu" at the bottom of the new window that is displayed, and then click "Scan" at the right side of the window. After a few seconds, the 3D map display will light up, showing a 3D network of copper ore. Now you must simply dig where you see copper ore and collect as much as you like.
Upon completing this quest, Chen will give you two sentry type 01 turrets, 100 bullets (required to replicate) and the script to make them.