Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
The answer to the green screen when trying to play an app like HBOMax or Netflix is to clear the cache. To do this, you press the home button on your remote 5 times, then the arrow up button, then the rewind button twice, and the fast forward button twice. Then wait. After a minute or more, the set will restart, and your cache will clear. I found this on YouTube as follows: