Postpone does not support scheduling posts to Facebook Groups, and neither does any other third-party social media tool.
Why Facebook Groups are not supported
Meta removed third-party access to Facebook Groups in April 2024. Specifically, Meta deprecated the Facebook Groups API as part of its Graph API v19.0 release, permanently revoking the ability for external applications to publish to Groups on behalf of users.
This was an industry-wide change — no scheduling tool has been able to post to Facebook Groups since then. For more background, see Meta cuts off third-party access to Facebook Groups (TechCrunch).
What you can do instead
Postpone supports scheduling posts to Facebook Pages, including standard posts, Stories, and Reels. See Scheduling Facebook Posts for details.
For Groups, posts will need to be published manually through Facebook directly.