This error means that URL and Domain in your Faecbook App don’t match your actual URL. It is very important that URL and domain in your Facebook App match your WP-Admin URL and domain. You must enter the URL and domain that you see in your WordPress Admin Panel. For example even if your visitors come to your blog at https://blog.nextscripts.com, but you manage your blog from https://www.nextscripts.com/blog/wp-admin/ – you must enter https://www.nextscripts.com/blog to the URL Field and www.nextscripts.com as the domain. Also please note that Facebook distinguish www. and non-www versions, so from Facebooks point of view nextscripts.com and www.nextscripts.com are two different domains.
You must have the correct domain and URL in 3(three) places in your Facebook app.
1. App Domains
2. Site URL
3. Valid OAuth redirect URIs. (Also please make sure that “Use Strict Mode for Redirect URIs” is set to “No”)
Please see Facebook Interface instructions