If you are getting this error it means, Either you do not have permission to post on this page now Or you have not assigned us a permission to share on your page during the authentication process.
We suggest, Login to your Facebook account => Go to Setting => Click on Apps => Search for SocialPilot and remove this app. Then do re-authentication in SocialPilot to add your page again. It will again start the authentication process.
Make sure you accept all the permissions this time to avoid this error.