The answer
Another process is using the port. Kill the other process or use a different port. On Mac/Linux, use lsof -i :PORT to find the process, then kill it. On Windows, use netstat -ano | findstr :PORT.
Common Node.js Setup Errors and How to Fix Them
How do I fix 'Port already in use' in Node.js?
More FAQs in Common Node.js Setup Errors and How to Fix Them
Node.js is not installed or not on your PATH. Install Node.js from the official site or use a version manager like nvm, then reopen your terminal so the PATH updates.
You are requiring a module that is not installed. Run npm install to install all dependencies from package.json, or install the specific package with npm install package-name.
Use a version manager like nvm so global installs do not need elevated permissions, and fix folder ownership for node_modules if it is wrong. Avoid using sudo, which masks the real problem and can cause more issues.
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