You need to gather the necessary materials to craft a chestpiece for your armor at a research station.
Craft the Pathfinder chestpiece Edit
Simply go to any Research Station and use the "Development" section to craft a Pathfinder Chest. You can find it in the "Ready To Develop" tab (default), or by going to the "Armor" tab and selecting the "Chest" subcategory.
Note: This can be done with any level of the Pathfinder Chest, and doesn't have to be completed by crafting the level I version.