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As per Facebook platform product changes and policy updates

Apps in public mode no longer allow their admins, developers, or testers to access permissions or features that normally require app review. This affects all apps built after May 1st, 2018, immediately. Apps built before then will not be affected until August 1st, 2018.

There are 2 options,
one is to submit your app for review(required permissions- manage_pages and publish_pages) and get it approved(See steps : https://help.xyzscripts.com/docs/social-media-auto-publish/faq/how-can-i-create-facebook-application/)
second is to use our pre-approved app .
(This is a hosted service and hence requires payment ,See steps : https://help.xyzscripts.com/docs/social-media-auto-publish/faq/how-can-i-use-the-alternate-solution-for-publishing-posts-to-facebook/ ).
You can use the free trail of SMAPSolution’s app for 1 month.
Within this time you can complete the fb app review or continue using our hosted solution.

Hope this helps.
Thank you.