I'm trying to upload an application to the Ubuntu Software Center and have encountered this error when uploading the .deb file:
The upload does not appear to be a valid click package.
What is the reason for this error?
Sounds like you were accidentally in the phone section, clicks are currently for Ubuntu Touch. Click on "Desktop" as shown here:
That should get you in the right section, also remember that you won't get e-mail's for notes on the submission, so check the status and feedback areas often.