There may be a variety of reasons why you see this error message after trying to send a message in the Community Management app.
Facebook doesn't specify the exact reason, but it can be one of the following:
Event post/Job posting/Review
You're trying to answer a comment under an event/job/review post, which isn't possible due to Facebook API limitations. Unfortunately, API sends us the comments, but it doesn't allow answers.
The comment is deleted
The page you are reacting as doesn't have all the necessary permissions for this action. Read more about permissions here.