Hello there, tsoclose. Thanks for reaching out and welcome to the Community. At this time, Alexa does not support playback through Napster. This means you will not be able to initiate playback of a Napster playlist. Once Alexa is able to support Napster it shouldn't be long before that is brought to Sonos.
However...
You can control playback (Play/Pause, Volume Control, etc.) with Alexa, once the playlist is playing.