Have you encountered this error message in your Community Management app?
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:
The person blocked messages from your Page (that's why you are not able to contact them via a private message)
The person deactivated/deleted their account
The person's account got restricted or disabled by Facebook
There's a technical issue on Facebook's side