Basically, you need to clear out the transport tokens. This can happen if you were to close out of Xcode while in the middle of submitting an app to iTunes Connect.
The token files now appear in the
Library/Caches/com.apple.amp.itmstransporter/UploadTokens/subfolder of the given user’s home directory. Which, honestly, is a better place for them anyway.
Delete any .token files in this directory.
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If you are unable to find the .token files, this is because they are hidden in Finder. To hide/show hidden files in Finder, use the following Terminal command (TRUE = UNHIDE, FALSE = HIDE):
defaults write com.apple.finder AppleShowAllFiles TRUE;killall Finder