JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post">
                    <fieldset>
                Atendente:&nbsp;
                <select onchange="habilitaBTN()" id="opcao">
                                <option name="ativo" value="--"/>--</option>
                                <option name="ativo" value="FUNCIONARIO1" />FUNCIONÁRIO 1</option>
                                <option name="ativo" value="FUNCIONARIO2" />FUNCIONÁRIO 2</option>
                                <option name="ativo" value="FUNCIONARIO3" />FUNCIONÁRIO 3</option>
                                <option name="ativo" value="FUNCIONARIO4" />FUNCIONÁRIO 4</option>
                                <option name="ativo" value="FUNCIONARIO5" />FUNCIONÁRIO 5</option>
                              
                            </select>
                        <br/>
                        <button id="avancar" type="submit" disabled="disabled" class="btn btn-primary pull-right"><a href="../../dashboard/questionario_posvenda/data_instalacao.php">Avançar</a></button>
                    </fieldset>
            </form>

JavaScript

function habilitaBTN()
            {
                var op = document.getElementById("opcao").value;
                if(op == "--")
                {
                    if(!document.getElementById('avancar').disabled)document.getElementById('avancar').disabled=true;
                }
                else if (!op == "")
                {
                    if(document.getElementById('avancar').disabled)document.getElementById('avancar').disabled=false;
                }
            }