Some upgrades require Resources specific to the type of upgrade you want to do. You can view the required upgrades from the upgrade screen. If you are missing any required resources, you can purchase them for gold.

As per the screenshot below, you can see both the resources and the cash required to perform the upgrade.

Aside from purchasing with Gold, resources can also be won from events and purchased from our Resource Shop which changes daily. Please refer to our Resource Shop in the FAQ for further information.