Can't complete Wonder

I have donated as much stone and bronze age resources that can be donated, but still have a way to go towards completion. There are three more resources that need donated, but those must be iron age resources, and although it says in a couple places that I have reached the iron age I can’t draw any iron age buildings nor trade for any iron age resources. What do I need to do? I’m stuck.

Hi,

Thank you for reaching out. Once you reach the iron age within a stage, you are rewarded with a limited building pack where you can draw iron age buildings from it.

Thanks,