You’ve got two options:

1 Import the file

import UserListSample from 'path/to/the/assets/UserListSample.csv';

export default function DownloadAnchor() {
  return (
    <a href={UserListSample} download="UserListSample.csv">
      Click here
    </a>
  )
}

Yor bundler should automaticaly bundle the file and adjust the href.

2 Place the file in the public folder

Given the snippet:

 <a href="/assets/UserListSample.csv" download="UserListSample.csv">
    Click here
 </a>

I conclude you should put the file in public/assets/UserListSample.csv; bundlers put everything from the public folder into the package as-is, so it should be available for download after you run the app. You need to put the right path in the code yourself.

3 If above doesn’t work

It’s probably bundler’s fault. Try building the app (probably npm run build) and investigate the contents of the built package – see if the file is there somewhere. If it’s not then it means it’s not bundled properly. Search for UserListSample in other files to find that anchor you made – see what the path is – does it actually point to the file?

You can also just share more code, including package.json and other files related to how you run your app. The CodeSandbox is a great tool for this (even if the sandbox might have some limitations disallowing to actually download files, a Proof of Concept can still be made).