When submitting form, I get “Cannot Post /”

I am building a reverse proxy, in it, there is a form box. It is supposed to get user input, and check if it is:

1. a url;
2. a url without the https:// header, which it adds;
3. or plain text, which is routed into a search engine.

Wen submitting the form, no matter what is in it, it returns “Cannot Post /” in a blank page. My index.html form is this.

<div class="search-container">
    <p>Join the <a href="https://discord.gg/3TTahctF">discord</a>!</p>
    <form id="fs" method="POST">
        <input id="is" class="main-search" placeholder="A World, Uncensored" type="text" />
    </form>
</div>

In my index.js, which is connected, this.

window.addEventListener('load', () => {
  navigator.serviceWorker.register('../sw.js?v=4', {
    scope: '/a/',
  })
})

const form = document.getElementById('fs')
const input = document.getElementById('is')

if (form && input) {
  form.addEventListener("submit", async event => {
    event.preventDefault();
    processUrl(input.value, "");
  });
}

function processUrl(value, path) {
  let url = value.trim()
  const engine = localStorage.getItem('engine')
  const searchUrl = engine ? engine : 'https://www.google.com/search?q='

  if (!isUrl(url)) {
    url = searchUrl + url
  } else if (!(url.startsWith('https://') || url.startsWith('http://'))) {
    url = 'https://' + url
  }

  sessionStorage.setItem('GoUrl', __uv$config.encodeUrl(url))
  const dy = localStorage.getItem('dy')

  if (path) {
    location.href = path
  } else if (dy === 'true') {
    window.location.href = '/a/q/' + __uv$config.encodeUrl(url)
  } else {
    window.location.href = '/a/' + __uv$config.encodeUrl(url)
  }
}

function isUrl(val = '') {
  if (/^http(s?):///.test(val) || (val.includes('.') && val.substr(0, 1) !== ' ')) return true
  return false
}

First, I tried changing the form’s method, which gave me a similar error. Then, after doing research online, I came to this link. Although similar, it is focused on an express error, while mine is basic javascript (client side).