Error: Permissions error

This is a general error that Facebook may return when trying to reply from the service.
It can be related to various situations, so it is important to understand the details of how it occurs (what exactly you were trying to reply to, whether you reauthorized in the service with a different user, etc.)

In a particular case, the error occurs when trying to reply to a comment under someone else's post.
Unfortunately, the Facebook API does not allow replying to such comments from the service, but you can reply directly from the social network.