I've had the same problem on my roku streaming stick 4k for the last 10 or so days. Same problem on an old now tv smart box too. The only workaround is to press * on the remote then select tv guide to switch to a live channel or select search and typ...
Finally, it's now available. There is a Sky Sports Box Office app in the App Store on the NOW TV box. Events can be purchased online www.skysports.com/boxofficelive