We’re trying to get our extension signed – we have no need for it to be listed on AMO as we distribute it ourselves so I’ve set it as ‘Unlisted’.
However, it keeps being rejected as ‘Incomplete’, but I have no idea what’s incomplete about it. The extension has been available for 8+ years and has hundreds of thousands of users.
You add-on is incomplete because there are no pending or approved versions. The submitted version has been rejected. You should have gotten an email to the account that is listed in the developer hub. Also, you can see the reason for the rejection on https://addons.mozilla.org/en-US/developers/addon/remember-the-milk-for-gmail/versions/1734886 by clicking on “Show File History”.