This error often occurs due to issues in the deployment pipeline. Ensuring that your pipeline is correctly set up and that the application is deployed properly is crucial. It's recommended to review the deployment steps and make sure the pipeline correctly points to the necessary files and folders. Specifically, in the release pipeline task, the package folder path should be correctly mentioned, pointing up to the artifact file like a .zip file.
When deploying a React app, it's essential to create a production build using the 'npm run build' command, which generates a build folder with the necessary files. This build folder should be referenced while deploying the React app to Azure. Make sure that this step is correctly implemented in your deployment process.
Azure App Service needs a default document (like index.html) to serve as the entry point for your website. If there is no default page, you may encounter the permission error. You should add the default document in the project's root folder and set it as the default page in the App settings on the Azure portal. Additionally, check for any IP restrictions or authentication settings in the Azure web application that may be causing access issues.
If your App Service is hosted on Windows, ensure you have a web.config file to manage routing options for the IIS web server. For Linux hosts, after deploying the React app, you need to configure the use of pm2.