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When I clicked on the co-op at the beginning it showed 25. I am almost at 28 and still no option to build!
You don't build the co-op, you join one.. View the selected co-ops in "browse co-ops" or select the Co-op forum from categories. There are always players posting their co-op for those to join.
Also, when you get it, you can also create your own co-op (vs join an existing one). Invite others to join or just stay in it by yourself.
If you form your own co-op, expect to do between 30 and 50 orders a week to get the 200 extra favors. With 2 players it runs between 50 and 70. This is no problem for some with huge inventories and farms. Also there are NO shared orders and no one to help you with requests.