as official tutorial said:
Be sure to send the Pug runtime (node_modules/pug/runtime.js) to the client in addition to the template that you just compiled.
but actually cannot find out in node_modules/pug folder.
in addition, i searched each sub-folders and each files, still no.
can anyone help on this?

Some parts of the official documentation and tutorials haven't been updated in a while. This issue however has been discussed over at the projects GitHub repository. You can read through it here:
runtime.js not existing. #2547