Error: “Production Center is invalid, become a farmer to continue to use it”. But how?

The production center is the Farmer class specific building. You need to change your class to Farmer. You will lose access to the specific building for your current class though when you do.

You can change class every 3 levels that you upgrade the base building or if you have a class change item. If not you can buy a class change item from the store.

Tap on your base building and select 'Class' option to see you current class and the options for changing.