JavaScript enable button if the two select option has value

You need to get the change event from your select then check if they are empty or not.

https://api.jquery.com/on/

https://api.jquery.com/prop/

$('.select1').on('change', function() {
  checkSelects($(this).val(), $(".select2").val());
});

$('.select2').on('change', function() {
  checkSelects($(this).val(), $(".select1").val());
});

function checkSelects(select1, select2) {
  let buttonProp = select1 != "" && select2 != "" ? false : true;
  $('button').prop('disabled', buttonProp);
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
Select 1
<select class="select1">
  <option></option>
  <option value="test">Test</option>
</select>
<br><br>
Select 2
<select class="select2">
  <option></option>
  <option value="test">Test</option>
</select>
<br><br><button disabled="disabled">Button</button>

your code is perfectly fine except a few minor issues. First of all on these two lines, remove the space after both id

var level = document.getElementById('level ')
var savebtn = document.getElementById('savebtn ')

And on the line that you're checking if either one is empty, that should be an OR statement

 if(language.value == '' && level .value == ''){
     //Since you're using an "AND" (&&) statement, 
     //it is evaluating to true on when both selects 
     //have an empty value. If either one has any 
     //value, the statement is returning to false, and 
     //`savebtn.disabled = true;` part is not getting executed
     savebtn.disabled = true;
 }

With all that being said, here's how I would rewrite the changebutton function

   function changebutton(){
     var language = document.getElementById('language')
     var level = document.getElementById('level') //Removed additional space after level
     var savebtn = document.getElementById('savebtn') //Removed additional space after savebtn
     if(language.value == '' || level.value == '') //Changed AND (&&) operator to OR (||) operator 
     {
       //gets executed if language or level has empty value
       savebtn.disabled = true;
     }
     else
     {
       savebtn.disabled = false;
     }
  }