Apple has started to require a different launch image structure for new apps/updates, using an Xcode storyboard instead of static PNG files. The deadline for this is today, and unfortunately we had not yet taken the steps to enable this support in Build Service.
Thus, unless Apple has secretly extended the deadline still, you cannot submit new Composer-built iOS apps or updates to existing iOS apps to the Apple App Store, until we fix the issue.
We are working on a hotfix right now, and will post details in this thread ASAP. Sorry for the inconvenience, no excuses for not tackling this earlier.