We have been receiving the same error from itunesconnect.apple.com when trying to upload a new ipa created using Xcode 8.0 (8A218a) macOS Sierra 10.12. We tried every solution we could find while waiting for Apple support to respond and nothing worked. This included creating builds with 8.x deployment targets and 9.x deployment targets.
Apple support finally responded that the issue was due to 2 images created using the P3 display gamut, which didn’t make much sense since the error response for that issue is usually ERROR ITMS-90682 Assets.car can’t contain 16-bit or P3 assets if the app supports iOS 8 or earlier.
We updated the 2 images they flagged and tried to upload again and we received the same error message about having used a beta version of Xcode.
We finally found a solution by using Xcode to upload the build to the app store instead of the website and it worked!
Apparently there is a bug with itunesconnect.apple.com, which misidentifies builds created using Xcode 8 as being created with a beta version of Xcode.