The generated dynamic frameworks must be embedded
into your app to be loaded at runtime.

Important:
  Plugins might produce static or dynamic frameworks.
  Static frameworks should be linked on, but never embedded.
  If you embed a static framework into your application,
  your application is not publishable to the App Store
  and fails with a
  Found an unexpected Mach-O header code archive error.

After linking the frameworks, you should see them in the 
Frameworks, Libraries, and Embedded Content
section of your target’s General settings. 
To embed the dynamic frameworks
select Embed & Sign.

They will then appear under Embed Frameworks within 
Build Phases as follows:

You should now be able to build the project in Xcode using ⌘B.

Option C - Embed application and plugin frameworks in Xcode and Flutter framework with CocoaPods