This message means that there already is a Customer Account in your Shopify Store with the same email address which is used in your FB account, and this account was created directly with email/password. In that case our app can not log you in (as in this case we'll need to change account password to temporary one-time password according to app architecture, and customer will not be able to login with email/password anymore), and can not create new account, as email should be unique according to Shopify logic.
In other words, if customer would like to login via social network account, he should create his account via social network also. And vice versa, if he created his account with email/password, he'll need to use email/password to login.