Overall.

• Speaker firmware must be 665 or higher
• Only the newest Zipp and Zipp Mini speakers support this feature
• Remember to register your speakers in Libratone App

Alexa skill is connecting to the speaker, but when i try to play the music on the speaker Alexa says, "<speaker name> is in a mode that does not accept requests"

The speaker must have something to play otherwise you get that response.

If a user can set a Spotify Connect playlist on the speaker or some other audio source working on it, this command should work then.