Block a button if an input is empty

If what you need is that, once the approved button is selected and the input with id 'fee' has no value, it is disabled, you would need the following:

For one thing, in your HTML code , you would need to add the property onchangeto the input #fee, so that you could catch the event when its value changes:

<div>
    <label for="status">Estado del proceso</label>
    <select name="status" id="status" onChange="statuses()">
        <option value="pendiente">Pendiente</option>
        <option value="aprobado">Aprobado</option>
        <option value="rechazado">Rechazado</option>
    </select>                    
</div>
<div id="honorary" style="display:none;">
        <input type="number" name="honorary" id="fee" onchange="disableUpdateIfFeeIsEmpty()">
    </div>
</div>
<input type="submit" value="Editar" id="update" class="btn btn-success" >

On the other hand, in your Javascript code , you would need a function that evaluates if the input #feecontains any value and enables or disables the button #update:

function statuses() {
  var x = document.getElementById("status").value;
  if ( x == 'aprobado') {
    document.getElementById("honorary").style.display = "";

    disableUpdateIfFeeIsEmpty();
  }else{
    document.getElementById("honorary").style.display = "none";
    document.getElementById("update").disabled = false;
  }
}

function disableUpdateIfFeeIsEmpty()
{
  if (document.getElementById("fee").value == '') {
    document.getElementById("update").disabled = true;
  } else {
    document.getElementById("update").disabled = false;
  }
}

Part 0

• I moved the listener outside the HTML code and by assigning it to a variable we can watch the eventchange
• Every time this event occurs we can obtain and assign to a variable the one valuethat each label optioncontains and that will serve us to do the filtering and subsequent logic

Part 1

• Through conditionals we first establish if the value taken from selectcorresponds to approved then:
  • We show the div that contains the input by changing its property to block, we achieve this through the variable with which we retrieve it
  • We check if the input we get for its id is empty then we disable it inputwith its propertydisabled
• In case the main condition at the beginning of the previous point is not fulfilled, then:
  • We return to hide the divone that contains the button
  • We re-enable the button by changing its disabled property to false

Code

    <div>
        <label for="status">Estado del proceso</label>
        <select name="status" id="status">
            <option value="pendiente">Pendiente</option>
            <option value="aprobado">Aprobado</option>
            <option value="rechazado">Rechazado</option>
        </select>                    
    </div>
    <div id="honorary" style="display:none;">
            <input type="number" name="honorary" id="fee">
        </div>
    </div>
    <input type="submit" value="Editar" id="update" class="btn btn-success" >
    
    <script>
      let opcionSeleccionada = document.getElementById("status")
      let boton = document.getElementById("update")
      let botonOculto = document.getElementById("honorary")
      let cajaTexto = document.getElementById("fee")
      
      opcionSeleccionada.addEventListener("change", () => {
        let seleccion = opcionSeleccionada.options[opcionSeleccionada.selectedIndex].value
        
        if (seleccion === "aprobado") {  
          botonOculto.style.display = "block"
          if(cajaTexto.value === "") {
            boton.disabled = true
          } 
        } else {
          botonOculto.style.display = "none"
          boton.disabled = false
        }
      })
    </script>

As a little advice, do not mix the JS code directly in the HTML, it would be more convenient to add actions to the tags, retrieving them through their id or classes and assigning them to variables.

Initially your button should be disabled:

<input type="submit" value="Editar" id="update" class="btn btn-success" disabled>

And then you can do this:

const fee = document.getElementById("fee");
const boton = document.getElementById("update");

function statuses(){
     //Sigue igual tu código aquí
}

function disableIfNotAble(e){
     if(e.currentTarget.value == ""){
        boton.disabled = true;
     }else boton.disabled = false;
}

fee.addEventListener("input", e => disableIfNotAble(e));

You can disable a button using the disabled attribute, so in a few words what is done there is to extract the elements that interest us, add a listener event to our input, the listener event is of type input (it is executed every time the value of the input changes), then, in the disableIfNotAble function, what is done is to check if the input has content, if it does, we set the disabled to false, but if not, we block the button, setting the disabled to true.