HTTP error 405 method not allowed

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Javascript

HTTP

Hi, I have a PWA application with push notification functionality. I am still new to this and have just recently added the push functionality into my app. I am referring to Get started with Progressive Web Apps - Microsoft Edge Development | Microsoft Docs[^] for my learning step.
I tried to run the app in Chrome and i encountered the error as same as title above in F12 development tools. Full error is as below:

POST http://localhost:portNumber/register 405 (Method Not Allowed)

.
I have install the web-push library as guided by the link above and have the following codes in the index.js file provided by the library installed:

router.post('/register', function (req, res) {
    // A real world application would store the subscription info.
    res.sendStatus(201);
});

At first i thought the codes in above link only support Edge but i do not think this is the case here as the following codes is executed after a promise is returned:

navigator.serviceWorker.ready
    .then(function (registration) {
        //do something with VAPID keys
    }).then(function (subscription) {
        // Send the subscription details to the server
        fetch('./register', { //error occurs here
            method: 'post',
            headers: {
                'Content-type': 'application/json'
            },
            body: JSON.stringify({
                subscription: subscription
            }),
        });

What I have tried:

1. Change the script by referring to another source but to no avail.
2. Search online for error 405 solution seems to be permission issue but could not pinpoint a root cause here.
3. Thought is path issue but in fetch('./register') but this is not the case.