Resource Marts help you generate resources in your cities, even when you’re not playing!
You’ll need to collect 10 purple Local Workers to build a Resource Mart. Make sure to check each level description to see which levels will give you Workers when you complete them.
Once you have all 10 purple Local Workers collected, you should be able to build the Resource Mart and pick a resource to generate.
NOTE: You cannot change the type of resource your Resource Mart generates once you’ve picked one.