There is no such module error when importing Swift user environment

Question

I have a large and old Xcode project with lots of mixed Objective-C / Swift code.

If I wanted to add the iMessages extension, I found that I need to start with a framework for sharing code between the application and the extension.

I tested this in a new project following this Ray Wenderlich tutorial and everything worked perfectly

When I try this in my old Xcode project, I get to the import phase in my application code to import my custom infrastructure, but I get an error No such module .... I checked everything several times, checking all the import data for the user infrastructure with no luck.

Over the years, this project has been tuned for many build settings, and I assume this is a problem?

Can anyone recommend any build settings in an Xcode project that I can verify? Nothing seems unusual, but I have no ideas.