Before you can craft a item you need to get your headquarters to level 10 and you can build your tech center level 1 you will also need to gain parts to build your first item.
keep attacking encounters or you can buy parts crates these cost gold and will give you random parts only do this as i found out not worth it i bought 20 crates for almost 2000 gold and didn't get the rubber i required.
you will need to build a 9mm for part 17 of the campaign.