Hi @eric410637340!
This problem happens when there is a WordPress account with that email address that has the same type of social media account (in your case Facebook) already linked to that account.
Here is an example to understand it better:
You have a regular WordPress user with this email address: [email protected]
You log in to WordPress via the standard WordPress login. Then with our “link” social login button, you link your Facebook account, which has this email address: [email protected]
Afterwards, a Facebook user tries to log in with this email address: [email protected]
And since [email protected] already exists, we cannot create it, so we try to link this Facebook account to the user, but that doesn’t work either, since it already has one Facebook account already linked to it, and we throw that error.
You can fix this by either logging in with the linked Facebook account ([email protected]), or with the traditional way, then if you want you can unlink this Facebook account on your profile page, and link this new one if needed, or not and let autolink do that when the user connects with [email protected] Facebook account.
Also, there could be one more reason for this. In the case of Facebook, the IDs of the users are connected to the Facebook app:
https://nextendweb.com/nextend-social-login-docs/provider-facebook/#app_scoped_user_id
So if you have made a Facebook app previously for Nextend Social Login, logged in with your user, then made a new Facebook app, then the new one will not let that old Facebook user log in, as it is identified as a different user (has a different user id). If this is what happened, then what you could is delete the users from the [prefix]_social_users database table, so that they can log in once again.
