Thank you for using our forums. We are sorry to hear that you are having issues with using the Roku to stream. You would need to make sure that you have the newest version of the app for your phone. You can also try to uninstall and reinstall the app as well as the Roku itself. You would also need to make sure that you have a supported Roku model. The articles listed below should help you further.
Hope this helps,
The Sling Moderation Team