test.js

router.get(':/category', async (req, res) => {
  try {
    const category = await Category.findOne({ /* ... */ })
    const posts = await Post.findAll({ /* ... */ })
    res.render(/* ... */)
  } catch (err) {
    console.error(err)
  }
})

@kingdaro you can also do:

router.get('/:category', async (req, res, next) => {
  const category = await Category.findOne( /**/).catch(next)
  if (!category) { return res.sendStatus(404) }
  const posts = await Post.findAll(/**/).catch(next)
  res.json(/*stuff*/)
})

this assumes you have an error handler defined after your router(s)