It is possible that the channel is no longer being supported on Roku by the owner of the channel or with the multiple mergers of streaming providers that the channel is being discontinued. It is also possible that the channel was removed because of some issue with content.
It could also be a temporary issue as it appears there are a number of complaints about the app and they are trying to fix the issues and they temporarily removed the app so they can update and re-release it.
You could reach out directly to YES network customer support and see if they respond. They may be able to provide further information.