While attempting to link my Facebook account with my Plex account, I too happened on this issue, which results in a failure to complete the external account linking operation. The following is an explicit enumerations of the steps required to duplicate this issue:
Scroll down to the LINKED ACCOUNTS sub-section of the page and click the EDIT action for that context.
Assuming a Facebook account is not already linked, click the LINK action next to the square Facebook icon with the Your Facebook account is not linked text.
On the resulting page, continue the linking operation by clicking the CONTINUE WITH FACEBOOK button.
The resulting pop-up, which is a Facebook-hosted page that would normally allow you to accept the Plex application permissions in the context of your Facebook account (linking the two together), an error message is instead displayed:
Login Failed: You can’t use Facebook to log into this app or website because there’s an issue with its implementation of Facebook Login.
Please resolve this issue quickly, or (at the very least) remove the Facebook linking functionality. Thanks for this amazing project; I don’t know what I’d do without my PMS.