I got the same error when I submitted my app with a watchkit extension.
This is the solution that worked for me :
Deleted the AppIcon Image set from my Xcode project Images.xcassets with all the icons that I added.
Then recreate the AppIcon Image set from + button below when you select the Images.xcassets file. You can do this by clicking the plus button then select New App Icon.
A new AppIcon Image set will be created. Now, add all the required icons that you app support.Make sure they are in the PNG-24 format.
Finally, I selected my app name Target and under General tab I scrolled down to the section App Icons and Lunch Images and selected the newly created AppIcon images set in the App Icons Source
Clean your project and rebuild.
This solution worked for me and removed the errors.