Check the limitations in the FAQ’s. Bluetooth is disabled when Eras are used as surrounds.
Can I use Bluetooth as soon as I plug in the speaker?
Era 300 must first be set up on WiFi using the Sonos app. Following setup, you can easily pair a Bluetooth device by pressing the button on the back of the speaker. Please note that if you use a pair as rear speakers for home theater surround sound, Bluetooth will be disabled.
