We recently ran into this issue and I found that somewhere along the way the build script was removed. Adding it back in with the following fixed the issue for us:
./Crashlytics.framework/run <your_api_key> <build_secret>
Note : When using Cocoapods you will want to us the following instead of the above (source):
./Pods/CrashlyticsFramework/Crashlytics.framework/run
Adding a build script:
- To add a Run Script Build Phase in Xcode 6, select your application target in your project, then select "Build Phases".
- Click the small "plus" icon and select "New Run Script Build Phase".
- You should now see a Run Script section in the middle of your Build Phase options, as shown above.
- Inside the body of the Run Script Build Phase, paste in the script.`
The above quote comes from the Crashlytics visual tutorial, referenced in this post.