When developing a project with React, running npm start in your terminal launches your application in a web browser. However, there may be instances where the application fails to launch, accompanied by an error message in the terminal that states:

Something is already running on port 3000.

This means that another program is running on the same port, preventing your application from starting. Only one program can use a specific port at a time.

To resolve this issue, simply run the following command in your terminal:

npx kill-port 3000

This command will remove any program that might be using the same port. By doing so, it frees up the port, allowing you to start your application without any conflicts.

Afterward, rerun npm start, and your application will successfully launch.