Hi @Aqil_Abbas_Khan,
Sorry for inconvenience. It looks like on the page of Login Provider “Callback URL” is displayed in old format. For your app URL should use generated domain. I have created an internal ticket BKNDLSS-28588 to fix this problem.
Could you please specify this URL https://classicscissors.backendless.app/api/users/oauth/facebook/authorize in “Valid OAuth Redirect URIs” section and try once more? All other entries can be removed from this section.
Regards, Andriy