You can't. resolve_object in the API needs a full URL - something you could also put in a browser to uniquely identify an object - a user, community, post, or comment, but not a modlog entry.
You'll have to get the mods / admin at blahaj to flip the ban again.
Other instances have recorded that particular unban, so it's possible that - at the time it was sent, lemmy.nz rejected it because the community didn't have any local subscribers. So, considering the activity, it would be a lemmy bug. Maybe not though, maybe lemmy.nz was just down at the time. Anyway, it's showing 1 local subscriber now, so assuming that's a different account, another attempt should be acknowledged.