Why your Facebook Group URL won't connect
Curator cannot pull posts from Facebook Groups, so a facebook.com/groups/... URL will not connect as a Facebook source. Meta has deprecated the Facebook Groups API and removed it from all versions of the Graph API, which means displaying content from a Facebook Group is no longer possible. Facebook Pages don't have this restriction.
What to do
Check whether your organization has a Facebook Page (not a Group). If it does, connect that Page instead.
Follow the standard setup steps in Adding Facebook Source: on the Organize page, click Add Source, select Facebook, choose the Facebook Page source type, and click Continue with Facebook to authorize with an account that is an administrator of the Page.
If the Group is all you have, create a Facebook Page for your public-facing content and connect that Page as your source.
Facebook Page and Page Mentions sources are available on all plans that support Facebook sources, and each Page source counts as 1 source toward your plan limit. For other Facebook connection issues, see Resolving Connection Problems.