SNY will only be available via a paid subscription to a service that carries it, so that SNY can get paid. The Roku Channel is not a paid subscription service.
Here's an article re how to get SNY now that YouTubeTV is dropping it: https://www.techradar.com/streaming/watch-the-mets-on-youtube-tv-youll-have-to-look-elsewhere
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X