If you are seeing the “Builder code not found” on one of your entries while using Builder for Shopify:
this means that the Builder editor cannot find any Builder code on the current page you are trying to view.
This could just be because the section code was never installed, or because you recently changed themes. Let’s say you are working on a Collection page top section and you see this error. To retry auto installing the section code, go to the content list view in builder, click on +New, choose collection page, then go through the flow and choose to create a new top section.
If that does not work (maybe because you are using alt templates for your collection template), you can manually install the code. To manually install the builder section in the proper location, go to the models tab in Builder and select the model you want to use (e.g. Collection page top), copy the code that looks like:
{% comment %}
Include this anywhere in your liquid code
Alternatively, you can use symbols to include this content in any other Builder content
https://builder.io/c/docs/symbols
{% endcomment %}
{% include 'model.collection-page-top.builder' %}
and then paste it into wherever you want it to show up. In this case that is probably at the top of the collection.liquid file, since you want the section to show up at the top of your collection page.
If you update themes frequently, make sure to transfer all builder files when you do code pushes. Here is a guide: How to do a code push and switch Shopify themes. If instead you want to control all theme modifications in your own source code and do not want Builder pushing any automatic theme updates, you might want to explore our setting to disable auto section installation flows: How do I limit theme studio to only create landing pages and sections?, or even setting your models to be client side only (outlined at the top of this post: How do I target by query string parameters in Shopify using theme studio?).