JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<head>
<script type="text/javascript">
function disableField(val) {
    console.log(val);
    var cantidadCopias = document.getElementById("cantidadCopias");
  if( val.length > 0  ) {
    cantidadCopias.disabled = true;
  }else{
    cantidadCopias.disabled = false;    
  }
}
</script>
</head>
<body>
<label> <span>Valor final:</span>
  <input type="text" class="input_text" name="valorFinal" id="valorFinal" onkeyup="disableField(this.value)"/>
</label>
<label> <span>Cantidad de Copias:</span>
  <input type="text" class="input_text" name="cantidadCopias" id="cantidadCopias"/>
</label>
</body>
</html>