Facebook authentication

Unknown Error, I found this topic https://www.nopcommerce.com/boards/t/46804/facebook-login-unknown-error.aspx, i have a question where to add new class? NopCommerce 3.9/

geldi wrote:

Could you more explain your question?

sohel wrote:

I want to integrate Facebook authentication, I fill App ID/API Key and App Secret, when I sumbit ok during connecting to FB account, the page redirecting to login page with "Unknown error"((

geldi wrote:

Make sure your App ID/API Key and App Secrets is correct.

If you have source code then debug, otherwise you can see the error details,see how you find ==>http://docs.nopcommerce.com/display/en/Logs

App ID/API Key and App Secrets is correct 100%.
Now, when I click on Fb button, I have "Unkwown error" immediatelly on login page.
In Logs page, I do not have mistakes related to auth...