using functions folder syntax, how do I reference variables?

18:09:58.899    ✨ Success! Uploaded 0 files (2 already uploaded) (0.36 sec)
18:09:58.899    
18:09:59.097    ✨ Upload complete!
18:10:02.665    Success: Assets published!
18:10:07.438    Error: Failed to publish your Function. Got error: multipart uploads must contain a readable body_part, main_module, or assets

 - 21:33:08.188    ✨ Upload complete!
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21:33:10.898    Error: Failed to publish your Function. Got error: multipart uploads must contain a readable body_part, main_module, or assets

export function onRequest(ctx) {
  const { env } = ctx;
  // Do whatever
}

{
  "index.html": "index.html",
  "styles.css": "styles.css",
  "script.js": "script.js",
  "images/logo.png": "images/logo.png"
}