If you see the message Audio Engine Not Available on the interface of the library, it is due to the audio engine not being available on your machine. In order for Kontakt to properly load the library, the audio engine needs to be running.

To resolve this in Kontakt standalone, make sure that your computer's sound is set to an available audio output, or any audio interface connected to your computer. In the Settings panel of Kontakt, make sure the right output is selected under Settings > Audio > Device.

In the Kontakt plugin, make sure the track is not disabled and that the audio output driver is active.