Shopify get Customer Metafields

UPDATE: I have this Private API request to get a customer metafield data that sends a request to the shopify admin.

const {getAccessToken} = require('./auth')
    const request = require('request')
    const {shopFromPermanentDomain} = require('./helpers')

const getCustomerMetafields = ({accessToken, id, shop}) => new Promise((resolve, reject) => {
  request({
    url:`https://${shop}.myshopify.com/admin/customers/${id}/metafields.json',
    headers: {
      'X-Shopify-Access-Token': accessToken
    }
  }, (error, response, body) => {
    const {errors} = JSON.parse(body)

    if (response.statusCode === 200) {
      const { metafields } = JSON.parse(body)
      resolve(metafields)
    }

    reject({error: errors, status: 500})
  })
})

const getCustomerMetafieldsResponse = (req, res) => {
  const {id, permanentDomain} = req.body

  if (id && permanentDomain) {
    const shop = shopFromPermanentDomain(permanentDomain)

    getAccessToken({shop})
      .then(accessToken => getCustomerMetafields({accessToken, id, shop})
        .then(meta => res.json({
          meta,
          status: 200
        }))
      )
      .catch(({error, status}) => res.json({error, status}))
  } else {
    res.json({error: 'Missing params', status: 500})
  }
}

module.exports = getCustomerMetafieldsResponse

I make this request from the front-end to my API with the following request.

const getCustomerMeta = ({
   id,
   permanentDomain
}) => new Promise((resolve, reject) => {
  post({
    params: { email, permanentDomain, task: 'get-customer-meta' },
    then: ({ error, id, state, status }) => {
      if (status === 200) {
        resolve({ id, state })
      }

      reject(error)
    },
    url: '/apps/spoke'
  })
})
    getCustomerMeta({
       id, // 98303739294 (Customer ID)
       permanentDomain // "store.myshopify.com"
    })

When making this request, I get the following request error:

 500 (Internal Server Error)

VM706781:8 Uncaught SyntaxError: Unexpected token < in JSON at position 7
    at JSON.parse (<anonymous>)
    at XMLHttpRequest.l.onload

I then want to GET the customer metafields data so that I can populate the front-end with the data gathered.

Thanks!