You need to be calling a stream in your snapshot URL that is active in the transcoder. it looks like you're trying to use stream "15". Typically, when calling a stream from the transcoder, you would have created a label for that stream in your transcoder template (i.e. "_360p" which would be "15_360p" in your URL). Have you successfully setup the Transcoder? For more information, see this tutorial on
How to set up and run Wowza Transcoder AddOn for live streaming.
If this doesn't help, please create a support ticket by sending an email to
email@example.com. Include a copy of your [install-dir]/conf and /logs folders as a .zip file (or other archive) so we can take a closer look at your configuration and server activity. Also, please include a reference to this post.