JSFiddle - React, Tailwind, and code Playground

HTML

<form id="phonetypeform">
      <select name="porting-p1" class="dropdown">
<option value="" class="disablenext">Please select an option...</option>
<option value="1" class="enablenext">I want to keep my current phone number</option>
<option value="2" class="enablenext">I want to choose a new number</option>
        </select>

</form>

<button class="nextbutton">Next Step</button>

JavaScript

$(document).ready(function() {

//Start Buttons
    $(".nextbutton").button({ disabled: true });
    
    $('.dropdown').change(function() {
        if ($('.dropdown').val() == 0) {
          $(".nextbutton").button({ disabled: true });
        } else {
          $(".nextbutton").button({ disabled: false });
        }
    });

});