Your app uses or references the following non-public APIs:
itemKey
The use of non-public APIs is not permitted on the App Store because it can lead to a poor user experience should these APIs change.
Next Steps
Please revise your app to remove any non-public APIs. If you have defined methods in your source code with the same names as the above-mentioned APIs, we suggest altering your method names so that they no longer collide with Apple's private APIs to avoid your application being flagged in future submissions.
Apple says,
For a example, it' in the LDAdServiceChartboost.h.
One thing is strange that I use this API in my oldest version APPs, but threre're no rejects with this issue. I told Apple wondering why.
Apple replies,