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

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

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

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

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.