You need the file Assets.car under your Clean.app. Also you need the proper AppIcon*.png under the root. A good way to verify that they exist is
- take your ipa, rename to zip .
- extract the .zip file and you should see Payload subdirectory
- Browse under Payload, show package contents of Clean.app
- When looking under Directory you should see AppIcon pngs, and the respective Assets.car.
If Assets.car is not created properly and loaded in the Payload of your .app, something in the build script is causing it to error. I had the same issue when I migrated from 4.17.2 to 4.18.0. I have both Xcode 8.3.3 and Xcode 9.0.0 running on my build machine, running both versions of UE4. I edited my iOS settings in Unreal Engine, added the marketing 1024x1024 png, changed the Build version to 11.0, and Metal 2.0, required a restart of UE4 editor. When I built this time, voila those missing files were miraculous under the .app Payload!
I don’t know exactly what I did, but toggling those settings seemed to work. I toggled back to iOS 10.0 and Metal 1.2, and it seemed to build just fine, and upload via Application Loader into TestFlight without the error notification.
Here is the .app root listing of files: