Receiving 404 error on Post Request using Postman MERN Stack
The issue that you have is here: app.post("api/users/login", (req, res)=>{. Your path does not start with a / which means that your express app doesn't expose that path at all for your clients (postman) to use.
How can you tell what's going on:
The response code is 404 which means right away that the resource does not exist. So the first thing you look at is the paths. The one in postman doesn't match the one in express.