Usually, the Custom Liquid section is available on your Shopify theme customization; however, if you cannot find it on the section list, you can follow the guide below to create Shopify Custom Liquid section for your theme.
– In your Shopify admin panel, go to “Themes” below the “Online Store” section
– Click “Edit Code” in the dropdown menu next to the “Edit theme” button of the current theme

– Click Add a new section and give it the name “custom-liquid-section” in the sections directory.

– Hit “Done”
– Add the code below to the section, and then choose “Save” on the right side.
{{ section.settings.custom_liquid }}
{% schema %}
{
"name": "Custom Liquid",
"settings": [
{
"type": "liquid",
"id": "custom_liquid",
"label": "Custom Liquid",
"info": "Add app snippets or other liquid code to create advanced customizations."
}],
"presets": [
{
"name": "Custom Liquid"
}]
}
{% endschema %}
Now, the Custom Liquid section is available in your theme editor. Do step-by-step to add it to your Shopify theme.