How do I get the correct access token for my Google Analytics Embed API app?

I have this code made by Google and I want to make server-side authentication, but the access token I get on https://developers.google.com/oauthplayground/ doesn't seem to work. What am I doing wrong?

When I use client-side authentication it works, but I need to show the charts without login. And of course, I deleted the access token just to post.

<script>
gapi.analytics.ready(function() {

  // Step 3: Authorize the user.

  var CLIENT_ID = 'Insert your client ID here';

  gapi.analytics.auth.authorize({
  serverAuth: {
    access_token: ' '
  }
});

  // Step 4: Create the view selector.

  var viewSelector = new gapi.analytics.ViewSelector({
    container: 'view-selector'
  });

  // Step 5: Create the timeline chart.

  var timeline = new gapi.analytics.googleCharts.DataChart({
    reportType: 'ga',
    query: {
      'dimensions': 'ga:date',
      'metrics': 'ga:sessions',
      'start-date': '30daysAgo',
      'end-date': 'yesterday',
    },
    chart: {
      type: 'LINE',
      container: 'timeline'
    }
  });

  // Step 6: Hook up the components to work together.

  gapi.analytics.auth.on('success', function(response) {
    viewSelector.execute();
  });

  viewSelector.on('change', function(ids) {
    var newIds = {
      query: {
        ids: ids
      }
    }
    timeline.set(newIds).execute();
  });
});
</script>