Can't load my plugin

Hi guys,

I’m new to Atlassian add-on development and I’m trying to follow this guide:

I created two text documents (json and html) and followed the guide. After that I set http server on port 8000 and try to access my documents via localhost.
Localhost loads but I can’t access my files (including from http://localhost:8000/atlassian-connect.json links).
Here is the error message from my cmd:

[Thu May 26 2022 16:27:11 GMT+0300]  "GET /atlassian-connect.json" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.5005.61 Safari/537.36"
    throw new ERR_HTTP_HEADERS_SENT('set');

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:470:11)
    at module.exports.ResponseStream.(anonymous function) [as setHeader] (/usr/local/lib/node_modules/http-server/node_modules/union/lib/response-stream.js:100:34)
    at Object.exports.(anonymous function) (/usr/local/lib/node_modules/http-server/lib/core/status-handlers.js:57:7)
    at (/usr/local/lib/node_modules/http-server/lib/core/index.js:339:22)
    at Readable.emit (events.js:198:13)
    at emitErrorNT (internal/streams/destroy.js:91:8)
    at emitErrorAndCloseNT (internal/streams/destroy.js:59:3)
    at process._tickCallback (internal/process/next_tick.js:63:19)

What am I doing wrong?