iOS: Cannot proceed with delivery: an existing transporter instance is currently uploading this package

If you have this error in Application Loader, 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.

/Users/<username>/.itmstransporter/UploadTokens/

Delete any .token files in this directory.

Open a console and type "rm ~/.itmstransporter/UploadTokens/*.token"