I'm trying to create a sign-up page where users can enter information, however, I'm having issues with "app.post" properly working. This is what I have so far:

const express = require("express");

const db = require("./dbConnectExec.js")

const app = express();
app.use(express.json());

app.listen(5000, () => {
    console.log('App is running on port 5000');
});

app.get("/hi", (req,res) => {
    res.send("Hello world.");
});

app.get("/", (req,res) => {
    res.send("API is running.");
});

// app.post();
// app.put();

app.post("/customers", async(req, res) => {
    // res.send("/contacts called");
    // console.log("request body", req.body)

    let nameFirst = req.body.nameFirst;
    let nameLast = req.body.nameLast;
    let email = req.body.email;
    let password = req.body.password;

    let emailCheckQuery = `
    SELECT customer_email
    FROM Customer
    WHERE customer_email = '${email}'`;

    let existingUser = await db.executeQuery(emailCheckQuery);

    console.log("existing user", existingUser);

    if(existingUser[0]){return res.status(409).send("duplicate email")};

})

When I attempt to add a user through Postman, for example:

{"nameFirst": "Robert",
 "nameLast": "Redford",
 "email": "rob@mail.com",
 "password": "asdfasdf"}

I end up with "Cannot GET /customers"