localized resource file entry in config.json

"localizedResources": {
  "HelloworldWebPartStrings": "lib/webparts/helloworld/loc/{locale}.js"  // Remove this line
}

web part entry in config.json

"helloworld-web-part": {
      "components": [
        {
          "entrypoint": "./lib/webparts/helloworld/helloworldWebPart.js",
          "manifest": "./src/webparts/helloworld/helloworldWebPart.manifest.json"
        }
      ]
    }