Xcode 10 offers two build system modes: new as default, and legacy. However, the new build system doesn't index the libraries correctly when targeting an external build system, which is used by embedXcode. 

  • For an existing project, please Set Xcode 10 Build System to Legacy before performing a Build target.
  • For a new project, perform a first Build target to prepare the project and then add your code.

So the build system needs to be set to legacy.

  • Call the menu File > Project Settings...,
  • Set Build System to Legacy Build System,
  • Click on Done to close the windows.

Once this parameter is set, the normal procedure Prepare the Project indexes all the libraries and provides the contextual help, as shown below.

The project preparation underlying utility performs that setting automatically. 

Let's hope the final release of Xcode 10 will fix that issue.