Plex DVR, Trying to watch Live TV but get error "Unable to tune channel (no parts)"

There may be several causes for this error. It could be that there was an earlier error or there may be timeouts. Best is to get logs with debug logging enabled for the first such error after restarting the server

So please ensure debug logging is enabled
See https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/

To clear error flags please restart the server at this point
and on first persistent failure to tune - please capture a screenshot and the server logs
See https://support.plex.tv/articles/200250417-plex-media-server-log-files/