Factory reset your Sonos Playbar and add it back to your Sonos system. The app will then guide you through the process of pairing the Roku TV remote using an optical audio cable connecting the sound bar with the tv.
I figured this out after wasting my time on Sonos Live Chat, who gave me instructions for a previous version of their app. I waited 35 minutes on hold with phone support and they told me my remote wasn’t compatible as it wasn’t infrared, when in fact it is IR. They then gave me the same bad instructions as Live Chat, i.e. click on “Home Theater” and then “Setup Remote”, which doesn’t exist on the new app.