removeClass JQUERY does not work

When you use the CSS selectors in jQuery, the . indicates that it is a class. But you do not need that point in the removeClass method.

On the other hand, your selector is incorrect, try without spaces between the classes:

function RemoveClass() {

  var selectCon;
  selectCon = $('.btn.dropdown-toggle.disabled.btn-default');
  $(selectx).removeClass('disabled');

}

the reason is because (using a simpler example)

'div .btn' searches for items with class btn that are within of a div

'div.btn' searches for items that are div and that have the class btn

PS: Have you thought about simply adding an id to the element?

To select (or filter by) the CSS classes of the same level, you have to remove the space, for example:

 <div class="btn-group bootstrap-select disabled form-control has-feedback-left">

would be .btn-group.bootstrap-select.disabled.form-control.has-feedback-left

 <button type="button" class="btn dropdown-toggle disabled btn-default"

would be .btn.dropdown-toggle.disabled.btn-default

If you put spaces, assume that it is a child, example:

 <div class"padre">
    <p class="hijo tio"><span class="nieto"></span></p>
    <p class="hijo"><span class="nieto"></span></p>

to select the <p> usas .padre .hijo to select the <span> usas .padre .hijo .nieto o .padre .nieto . Also if you are looking for .padre.hijo or .hijo .tio you will not find elements, the first <p> would be .hijo.tio o .padre .tio .

No spaces = same level, with spaces = one level down.

Going back to your example:

 selectCon = $('.btn.dropdown-toggle.disabled.btn-default');

or more generic

 selectCon = $('.btn.disabled');

var $elementos = $(".btn-group.bootstrap-select.disabled.form-control.has-
feedback-left")
$elementos.add(".btn.dropdown-toggle.disabled.btn-
defaul").removeClass("disabled")

You can use .add () to add another object to your variable and remove the disabled class

I recommend in order to always use an id or class to find objects in jquery easier for example id="btnClientes" and thus nothing more serious to put $ ("# btnClients"). removeClass ("disabled")

There is no point in doing a multi-element selector if you only want to remove the class disabled . Better iterates class disabled of the elements that contain it, others do not matter. I leave you an example

$(function(){

  $(document).on('click', '#btn',function(){
    
    $('.disabled').each(function(){
      $(this).removeClass('disabled');
    });
  
  })

})

.disabled{
  background-color: red;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input class="foo bar poo disabled">
<input class="foo  disabled">
<input class="foo poo disabled">
<input class=" bar poo disabled">
<input class="foo bar poo disabled">
<input class=" bar disabled">
<input class="foo bar poo disabled">
<input class="foo bar">
<input class="poo disabled">

<input type="button" id="btn" value="quitar">