As I am developing a website using React, I have generated the react application. However, I am facing a problem where I am unable to modify the App.js file.

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There could be a few reasons why you are unable to modify the App.js file in your React application. Here are some possible solutions:

1. Make sure that you have the appropriate permissions to modify the file. If you are working in a code repository or with other team members, you may need to be granted access or permission to modify the file.

2. Check to see if the file is locked or in use by another application. If you have another program or editor open that is using the file, you may need to close it before making modifications to the file.

3. Make sure that the file is not read-only. If the file is set to read-only, you will not be able to modify it. To change this, you can right-click on the file and go to properties, then uncheck the "Read-only" option.

4. If you have made changes to the file and are not seeing them reflected in your application, try clearing your browser cache or restarting your development server.

5. If none of these solutions work, you may need to uninstall and reinstall the React application, or try creating a new React project and transferring your files and dependencies over.

Hopefully one of these solutions will help you modify your App.js file in your React application.