JSFiddle - React, Tailwind, and code Playground

HTML

<label> <span>Valor final:</span>
  <input type="text" class="input_text" name="valorFinal" id="valorFinal" onkeyup="disableField()"/>
</label>
<label> <span>Cantidad de Copias:</span>
  <input type="text" class="input_text" name="cantidadCopias" id="cantidadCopias"/>
</label>

JavaScript

var disableField = function () {
  var state = document.getElementById("valorFinal").value.length > 0;
  document.getElementById("cantidadCopias").disabled = state;
};