In my case it was necessary to add WatchKit Extension to WatchKit App target - General - Frameworks, Libraries and Embedded Content WatchKit App target - Build Phases - Dependencies

In my case a cause of the problem was second structure created with a name of existed structure, but located in a different group folder. It is weird, but XCode didn't provided an appropriate description on that problem. Cache cleans didn't help. The problem was localised after the first build run where structure names duplication was outlined.

I've met the same problem working on iOS Custom Notification Extension. The problem has appeared after trying to add App Group for this extension. I've got the message: Provisioning profile "iOS Team Provisioning Profile: myXXXXXXXExtension" doesn't match the entitlements file's value for the com.apple.security.application-groups entitlement. I've checked the entitlements file definition - it had correct definition of the group I've tried to add. After reading response of @nwpeakbagger in this thread I've checked "Bundle Identifier" value in Info.plist of this module. This value was absent as well as "Bundle display name" I've defined values for these attributes: "Bundle Identifier" = $(PRODUCT_BUNDLE_IDENTIFIER) "Bundle display name" = myXXXXXXXExtension The problem has been solved. Many thanks to @nwpeakbagger and Apple Dev Community!