JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<input type="checkbox" name="convocati[]" value="A">A
<input type="checkbox" name="convocati[]" value="B">B
<input type="checkbox" name="convocati[]" value="C">C
<input type="checkbox" name="convocati[]" value="D">D
<select name="S1">
   <option value=""></option>
</select>
 <select name="S2">
   <option value=""></option>
</select>

JavaScript

$(function(){

$("input").change(function(){


 var values =[];
 
 $("input").filter(":checked").each(function(){
 values.push($(this).val());
 }
);
 var options= "";
 for(i=0; i< values.length;i++)
 {
   options+="<option>" + values[i] + "</option>";
 
 }
 console.log($("select[name=s1]").length);
 $("select[name=S1]").html(options);
 $("select[name=S1]").html(options);
});
});