The app icon set "AppIcon" has an unassigned child

And another cause for this error: difference in filename case on disk and in Content.json. They need to match.

In my case, the filenames on-disk were all lower-case and that's how they were referenced in Content.json. Then I got new images, they were uppercased, and I just dragged them into Xcode to update them. Now on-disk and Content.json were both referring to the files uppercased.

But Git didn't pick that change in case up, it just thought the files were updated and did not change the case. So on the build server, I ended up with lower-cased filenames, but Content.json was referring to them upper-cased. Needed to lower-case them on my system and fix the Content.json.

Select the unassigned image and delete it via delete button. Clean and Build.

Ran into this same error but solved it in a different way.

In the project navigate to Assets.xcassets/AppIcon.appiconset/Contents.json and open this JSON file.

Review and remove any unnecessary set of icons.

For me, it was a group of iPad icons (seeing that the application was only iPhone compatible, it was not needed).

Once removed, Clean - then - Build