To ensure the process of removing and re-adding the NBC channel is successful. Make sure to do the restart from the Settings menu by navigating to Settings > System > Power (If you do not see a Power submenu, skip to the next step.) > System restart.
If still no dice, please be advised that many channels on Roku are maintained by the channel provider themselves. Since the error only occurs on the NBC channel, that's an indication that there's likely an issue within that specific channel that needs to be addressed with an update from them. We would recommend contacting NBC support to report the issue and get help.