In order for the section to show up on your store’s page and for it to be editable in Builder, the builder section code needs to be installed correctly in your store’s theme code. You can do this manually, or by using the automatic flow that allows you to install a section at the top or bottom of your theme pages.
When installing automatically, Builder will install the proper section include code to your theme’s main template file for the particular page type. For example, if you choose to add a Builder section to the bottom of your product page, Builder will append the proper code to the product.liquid template file. This works great for most cases, but some stores use alternate templates for specific products or for even for all products. What this means is that the default template (product.liquid) will not be used when viewing products with an alternate template, meaning the Builder code will not be present, so you will not be able to preview or edit the section.
To fix this, either use the standard template for the product you want the section to show up on, or copy and paste the builder section code to the alternate templates that you want it to show up on. The builder code will look something like this:
{% comment %}
product-page-bottom model installed by Builder.io
You may modify this code at any time
{% endcomment %}
{% include 'model.product-page-bottom.builder' %}
Continuing with our product page example, you would look at the product.liquid template in the Shopify Theme Code Editor for your active theme, find the code that looks like the example code above, then copy and paste that to the alternate product template (e.g. product.alternate.liquid) that you want the section to show up on.