Real-time preview not working for unpublished drafts in Next.js with Sanity

const token = process.env.SANITY_API_READ_TOKEN;
    if (!token) {
      throw new Error(
        'A secret is provided but there is no `SANITY_API_READ_TOKEN` environment variable setup.'
      );
    }
    const client = _client.withConfig({ useCdn: false, token });
    const route = await client.fetch(
      groq`*[slug.current == $slug][0]{ "slug": slug.current, "type": _type }`,
      { slug: req.query.slug }
    );

    // If the slug doesn't exist prevent preview mode from being enabled
    if (!route) {
      return res.status(401).send('Invalid slug');
    }

if (!route) {
      return res.status(401).send('Invalid slug');
    }