404 error while sending data from React to Node?

I'm trying to send a React state to Node, the idea is that when a user inputs a postcode,

1. React finds the geological coordinates,
2. store it in a state
3. React sends this state to an open weather api in Node code,
4. Node then fetches the weather data and sends back to React.

I have done 1, 2, 4, but 3 gave me an error. Here is the Node code which needs to receive the React state:

const axios = require('axios');
const dotenv = require('dotenv');
dotenv.config();

module.exports = (app) => {

    app.get('/search-location', (req, res) => {
        res.send('This is the search location page')
    });

    app.post('/search-location', (req, res) => {

        let baseUrl = `http://api.openweathermap.org/data/2.5/weather?`,
            apiKey = `&appid=${process.env.REACT_APP_WEATHER_API_KEY}`,
            coordinates = `lat=` + coord[0] + `&lon=` + coord[1], // here I need the coordinates from React state
            apiUrl = baseUrl + coordinates + apiKey;
        axios.get(apiUrl)
            .then(response => {
                res.json(response.data);
                console.log(response);
            })
            .catch(error => {
                res.redirect('/error');
                console.log(error);
                console.log('search location error')
            });
    });
}

Here is the React method to send the state to Node (I'm using a dummy coord variable to test):

sendToNode() {
    
    let coord = {
        longitude: 50,
        latitude: -2.1
    }

    axios.post('http://localhost:4000/search-location', coord)
        .then((response) => {
            console.log(response);
        }, (error) => {
            console.log(error); // here is line 36 where the 404 error logged in console
        });
}

I have googled as much as I could, the above method seemed to be correct, but I'm getting the following error in Chrome console:

xhr.js:178 GET http://localhost:4000/error 404 (Not Found)
WeatherTile.js:36 Error: Request failed with status code 404
    at createError (createError.js:16)
    at settle (settle.js:17)
    at XMLHttpRequest.handleLoad (xhr.js:61)

Why does this happen and how could I fix it? Thank you in advance!

UPDATE I changed the Node code to:

......
app.post('/search-location', (req, res) => {
        let coord = req.body.coord;
        let baseUrl = `http://api.openweathermap.org/data/2.5/weather?`,
            apiKey = `&appid=${process.env.REACT_APP_WEATHER_API_KEY}`,
            coordinates = `lat=` + coord[0] + `&lon=` + coord[1],
            /* coordinates = `lat=` + `51.5842` + `&lon=` + `-2.9977`, */
            apiUrl = baseUrl + coordinates + apiKey;
......

Now I get this error in Chrome console:

POST http://localhost:4000/search-location 500 (Internal Server Error)
WeatherTile.js:36 Error: Request failed with status code 500
    at createError (createError.js:16)
    at settle (settle.js:17)
    at XMLHttpRequest.handleLoad (xhr.js:61)