Polk Command Sound Bar with Alexa built in

I have bought a Polk Command Sound bar with Alexa built in. I have set the system up but when I follow the instructions in the Polk instruction book "Talking to your Polk Command Sound Bar" the book says " you can also ask Alexa to control your sound bar using voice commands, just say "Alexa, enable the Polk Command Bar Skill". When I try this request Alexa answers "Sorry I don't know that". What have I done wrong?