If you are trying to send a message to any Facebook user and facing an error, we might have an explanation here. Even if someone has interacted with your page, sometimes, when you try to send them a message, Facebook gives you an error.
Before reading about these errors, please check if your Facebook profile is connected to Statusbrew and doesn't need authentication. Read: Best Practices For Keeping Social Profiles Linked To Statusbrew
Facebook error: (#551) This person isn't available right now. (code 551, subcode 1545041)
Generally, this means the user has opted out of receiving any communication from your brand page on Facebook. Facebook does not disclose any further information about this error.
Facebook error: (#230) Permissions disallow message to user.
If a Facebook user has blocked Statusbrew in the Apps Platform, you might not be allowed to send them a message in Engage. There is hardly any chance of this happening with you but if it does, the workaround is to message them via Facebook website/app.