A Facebook Permissions Error can happen if you're trying to re-share a FB post that can't be shared.
The post may not be shareable for these reasons:
The original publisher of the post may have deleted it
The privacy settings of the post prevent it from being shared
In order to re-share any Facebook content to your own profile or page, the content must be public.
Common Permissions Error Message
Permissions error – Could Not Share Post – Sorry, the post that you’re sharing couldn’t be loaded. It may have been recently deleted. Or it may have privacy settings that prevent sharing.
If you are the owner of the content, please check the privacy settings for the post, and ensure that it is visible to the public.
Sometimes there may be age-related, or country-related, restrictions which prevent the content from being fully public.