Thanks for letting us know about the issue you're experiencing.
If the issue persists from one channel after attempting the suggestions above, videos from other channels play fine, contact the channel provider's customer support team to report the issue and get help. Channels on Roku are maintained by the channel developer themselves. In this case, there's likely an issue within that specific channel that needs to be addressed with an update from the channel developer.
We appreciate your understanding.
All the best,
Roku Forum Moderator