On ArtStation, you can sign in with Facebook, as well as email/password. There are, however, a few things you should be mindful of:
Do not attempt to sign in with Facebook yet. Instead, go to your settings page and hook up Facebook from this page when you are logged in.
This happens because ArtStation tries to match accounts by email address, and your Facebook and ArtStation accounts do not have the same email address. With mismatching email addresses, there is no way that ArtStation can tell that the Facebook account belongs to a specific account.
To fix this:
This happens because at some point, you probably accidentally clicked "sign in with Facebook" and it created a duplicate account. To fix this, you must remove the duplicate account.
Simple, just sign up for a new account and click the Sign in with Facebook button instead of registering with your email/password.