So it looks like you have the New Facebook Page Experience - these new pages are still a bit wonky and need to be added to the group differently.
I'm going to share the process below of how to set this group up to post as your business page. (You can also schedule it as a personal group if you prefer.)
You can watch the full video tutorial below or continue reading for written instructions.
1. Make sure your personal profile is an Admin in the group - this is necessary for the next steps.
2. If you had created the group as your page (or linked it to your page after), you'll need to remove your business page from that group.
To do this: Switch to interacting as the page in that group, and just using the 'Joined' button to 'leave group'.
3. Switch back to interacting as your personal profile, and check the Group Settings:
- Under Set Up Group: ensure that it is visible (not hidden - it can stay private though)
- Under Manage Membership (or Participation): make sure that both Pages and Profiles can join groups:
4. Switch back to interacting as your business page.
It may be helpful to copy the group URL and paste the link into the browser so you can see the Join button. Click that Join button.
5. Switch back to interacting as your personal profile.
Check the Member Requests tab to approve your page as a member.
Once the group shows your business page as a member of that group, you should be able to schedule your party normally as a business page group.
Please reach back out if you have any problems with any of these steps!