JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<form action="" method="POST" id="postForm">
  <div class="row">
    <div class="col-md-9 register-right">
      <div class="tab-content" id="myTabContent">
        <div class="tab-pane fade show active" id="home" role="tabpanel" aria-labelledby="home-tab">
        <h3 class="register-heading">Register</h3>
          <div class="row register-form">
            <div class="col-md-12">
              <div class="form-group">
                <select class="form-control" name="buy" id="buyoption">
                  <option class="hidden" selected disabled value="">Please select what you want</option>
                  <option value="1">1</option>
                  <option value="2">2</option>
                </select>
              </div><label for="buy"><b>YOU </b></label>
              <div class="form-group">
                <select class="form-control" name="sell" id="selloption">
                  <option class="hidden" selected disabled value="">Please select what you to</option>
                   <option value="1">1</option>
                  <option value="2">2</option>
                  </select>
              </div><input type="submit" class="btnRegister" id="btnProceed" value="NEXT" /> </div>
          </div>
      </div>
      </div>
    </div>
  </div>
</form>
</html>

JavaScript

$(document).ready(function() {
  $('#btnProceed').attr('disabled', 'disabled');

  function updateFormEnabled() {
    if (verifyAdSettings()) {
    alert('good to go');
      $('#btnProceed').removeAttr('disabled');
    } else {
      $('#btnProceed').attr('disabled', 'disabled');
    }
  }

  function verifyAdSettings() {
    if ($('#buyoption').val() != null && $('#selloption').val() != null) {
      return true;
    } else {
      return false
    }
  }

  $('#buyoption').change(updateFormEnabled);

  $('#selloption').change(updateFormEnabled);
});