This Mostly happens if your Facebook Page is a private page, so the best way to investigate this is by
1) Sign out from your Facebook Account
2) Now Try opening the Facebook Page of your favorite browser
3) If your page doesn’t open and you get redirected to facebook login page, this means your page is private and Facebook API does not allow to access to private pages To solve the above issue, you need to change your Facebook Page settings so it’s a public page and accessible even when you are logged out of your facebook account. But for some reason if your page is accessible publically and you are still getting the above error then please share your Facebook Page URL with us on firstname.lastname@example.org so we can get this Investigated.