Basically the board permission might be set to private, which means it's not accessible by the APIs we use. Currently this will silently fail, so if you try to create a rule or enable/disable it fails without any error message.
We should display a validation error around board missing check, and check to ensure board permissions aren't set to private.
In addition to this we might also want to filter our private boards from the list to avoid selected in the first place, although we should also fix the validation error to bubble to the front-end, to ensure the error is seen if someone modifies the board permission to be private later.
Create a sprint board
Click edit filter then details
Click on Private to set permission to only display yourself
Create rule with Sprint Completed trigger and select that same board
You'll receive errors when trying to save that rule, in addition if you change the permission of an existing one then you'll be unable to enable/disable that rule silently failing again.