Hey there 
Sometimes you’ll find a slight delay in response when posting a support question late on Friday.
Your issue is not a WP Discourse, or a Discourse, one. It’s an issue caused by the fact that Google doesn’t allow OAuth2 logins in embedded browsers (e.g. Facebook’s embedded browser). You’ll find that a number of other websites have the same issue. For technical detail on the issue see here.
The solution is to use a normal web browser, instead of facebook’s web browser to use your Discourse instance, or perhaps allow for other login methods if you want to let your users use facebook’s web browser.