In rare cases, the app script may not be running in your theme. To resolve this issue, please follow these steps:

First click on "Edit code" in the dropdown to open the code editor for your theme:

There are 2 things to add to your theme:

First, on the left hand side, scroll to Snippet section and add a new snippet. 

Name it smartSize.liquid

Next, inside the file, copy the following content:

<script>
window.sizefox = window.sizefox === undefined ? {} : window.sizefox;
sizefox.shop = "{{ shop.permanent_domain }}";
{% assign t = template | prepend: '/' | append: '.' %}
{% if t contains '/product.' %}
sizefox.data = {
  collections: "{{ product.collections | map: 'id' | join: ',' }}",
  tags: {{ product.tags | join: ',' | json }},
  product: "{{ product.id }}",
  productname: "{{ product.title | replace: '"', '"' }}",
  type: {{ product.type | json }}
};
{% endif %}
fetch(https://server.smartsize.io/sizepopupversion?shopname= + sizefox.shop)
.then (response => response.json())
.then(json => {console.log("Using SmartSize popup version: ", json)

scripturl = json.url;
  const smartScript = document.getElementById("smartsizeScript");
  smartScript.src = scripturl
});
</script>

Then click save. You should have a file that looks like this (see below). Make sure that you SAVE it.