Failed to unpack .xap Windows App Certification Kit 3.3

I am trying to run a Silverlight 8.1 WP application through the Windows App Certification Kit and I get an error stating that the file is not a valid .xap file. I'm not sure why it is happening and not finding much about it other than one article that hints at a manifest issue. I am however able to take a dummy Silverlight 8.1 app and Certify that without error (it fails of course, but no error). The actual error is:

Failed to unpack 'xap file'. The file does not appear to be a valid xap package.

Does anyone out there know why this can happen or at least where in the manifest I might start looking?

The answer to this turned out to be an issue with the Microsoft SDK I was using. The way I figured this out is by attempting to upload the .xap file to the store even though I got that very vague error because I was still able to side load the application, so I knew to some degree the .xap file was valid. I got the following error.

The last two errors (2001) gave me more to go on, so I did some research and came across this SO post which pointed to an issue with the Advertising SDK. It turns out that the SDK I was using was adding 2 entries for some reason instead of telling me it was the wrong SDK. I used the one that came with VS 2013 Update 1, but for Silverlight, you need to use the one located here under Windows Phone 8.1 Silverlight Apps

After installing this package (make sure it is 8.1 and not 8.0), I referenced that SDK and all was well again.

Hope this helps someone else with this issue