JSFiddle - React, Tailwind, and code Playground

HTML

<form id="form">
    <label class="mb-3 mr-1" for="radio-ko" required>Portes (accessibles?): </label>

    <input type="radio" name="gender3" class="btn btn-sm btn-outline-secondary" value="Ok" required> Ok &#9989;

    <input checked type="radio" name="gender3" id="radio-ko" class="btn btn-sm btn-outline-secondary" value="Ko" required>Ko &#10060;<br>

    <textarea name="gend3" rows="4" cols="20" placeholder="Si KO préciser les manquants" required id="textarea-ko"></textarea>
  </form>

CSS

input {
  display: unset;
}

textarea { 
  height: 30px;
  width: 1000px;
}

JavaScript

const form = document.getElementById("form")
const koRadioElement = document.getElementById("radio-ko")
const koTextArea = document.getElementById("textarea-ko")

form.onchange = () => {
  const isKoChecked = koRadioElement.checked
  koTextArea.required = isKoChecked
  koTextArea.disabled = !isKoChecked
}