JSFiddle - React, Tailwind, and code Playground

HTML

<button id='btn_habilitar'>Habilita</button>
<button id='btn_desabilitar'>Desabilita</button>
<br>
<br>
<textarea id='obs' placeholder='Observação!'></textarea>

JavaScript

var textarea = document.getElementById('obs');
window.addEventListener('click', function(e) {
    var id = e.target.id;
    textarea.disabled = id == 'btn_desabilitar';
});