In April 2018, Facebook made changes to how third-party apps like Post Planner can access private, closed and secret groups.
"Currently apps need the permission of a group admin or member to access group content for closed groups, and the permission of an admin for secret groups. These apps help admins do things like easily post and respond to content in their groups. However, there is information about people and conversations in groups that we want to make sure is better protected. Going forward, all third-party apps using the Groups API will need approval from Facebook and an admin to ensure they benefit the group. Apps will no longer be able to access the member list of a group. And we’re also removing personal information, such as names and profile photos, attached to posts or comments that approved apps can access."
When you try to post to a private/closed/secret group, you may get this error if the group admin is not posting from their specific linked personal account in Post Planner.
For more on this error and how to fix it, see our article on Facebook Group Permissions.