If you are following a customization article or installation instructions from third-party apps or Shopify, you may be looking for a
Out of the Sandbox themes do not contain these files, but instead contain an
app.js.liquid file can be located by going into Online store > Themes > Actions > Edit code > Assets.
For more information on the template file structure for the Flex theme, check out our Development Guide.
If you require further assistance on integrating third-party code, we recommend reaching out to a Shopify Expert.