I have single distribution certificate in Apple Developer center, as well as single distribution certificate in Keychain, but two certificates in XCode Settings, and the 2nd on has status: Missing private key. I can't install dext, may be because of wrong certificate usage. Reloading XCode and reboot didn't help. 'Delete certificate' in popup menu is grayed.

I had the same problem with Apple NullDriver Sample project - for test application, calling device extension. the problem was solved after removal of com.apple.developer.driverkit.userclient-access key from application entitlements

The problem was solved by removal of all build files in copied project, keeping sources only

The problem was solved by adding path to __debug file into the list of project sources.

In my case 'Build Active Architecture Only' in Build settings was set to No both for Release and Debug, but in project.pbxproj ONLY_ACTIVE_ARCH was set to No for Debug, but Yes for Release, I had to correct project.pbxproj manually to get correct output. v.15.0.1