The error POST http://localhost:4000/auth/login 404 (Not Found) means your frontend is trying to access a backend route that doesn't exist or isn't set up correctly. First, make sure your backend server is running on port 4000. Then, check that the /auth/login route is correctly defined and mounted — for example, if you're using Express, ensure app.use('/auth', authRoutes) is set and authRoutes includes a POST /login handler. Also, confirm that you're not using a different base path like /api/v1/auth/login, in which case your Axios URL should match that. You can test the route with Postman or curl to make sure it's working independently from the frontend.