JSFiddle - React, Tailwind, and code Playground

HTML

<form action="teste.php" method="post">
    <fieldset>Sim ou não?
        <label class="radio">
            <input name="ativo" type="radio" value="SIM" />SIM</label>
        <label class="radio">
            <input name="ativo" type="radio" value="NAO" />NÃO</label>
        <button class="avancar" disabled="disabled" class="btn btn-primary pull-right" type="submit">Avançar</button>
    </fieldset>
    <fieldset>
        <select>
            <option name="ativo" value="">--</option>
            <option name="ativo" value="FUNCIONARIO1">FUNCIONÁRIO 1</option>
        </select>
        <button class="avancar" type="submit" disabled="disabled" class="btn btn-primary pull-right">Avançar</button>
    </fieldset>
</form>

JavaScript

var inputs = document.querySelectorAll('form select, form input');
for (var i = 0; i < inputs.length; i++) {
    inputs[i].addEventListener('change', verificar);
}

function buscaBotao(elInicial) {

    // ir buscar o field set mais correspondente
    var fieldset = elInicial.parentElement;
    while (fieldset.tagName.toLowerCase() != 'fieldset') fieldset = fieldset.parentNode;

    // ir buscar o botao
    var botao = fieldset.getElementsByTagName('button')[0];

    // devolver o elemento
    return botao;
}

function verificar(e) {
    var el = e.target;
    var value = el.value || el.options[el.selectedIndex].value;
    var botao = buscaBotao(el);
    botao.disabled = !value;
}