In the background it says:
Account Removal Status: failed (Hook denied execution of killacct: Failed to remove these domains: sub.domain.com (last error: 'API call failed: code 404, msg "{"detail": "Not found"}".'). )
Hook denied execution of killacct: Failed to remove these domains: sub.domain.com (last error: 'API call failed: code 404, msg "{"detail": "Not found"}".').