Why do I get “Error: You must have at least one unique attribute in each entity. Please set at least one attribute unique in..” when I try to generate my WPAS app?

To generate your app you must have at least 1 unique attribute in each entity in your design. To check this, go to entities page in WP App Studio and click on your entity.  You will see a table with:  "Name,  Label, Type, Req, Unique, Created, Modified" columns for all your attributes. You should have at least one of your attributes with Y  value for Unique column.