JSFiddle - React, Tailwind, and code Playground

HTML

<select onchange="displaySubCategory()" onchange="ChangeSubCategory()" id="item-category" required>
          <option class="blank-condition-value" value="" disabled selected>Select</option>
          <option value="Antiques">Antiques</option>
          <option value="Arts & Crafts">Arts & Crafts</option>
          <option value="Baby">Baby & Nursery</option>
          <option value="Books">Books, comics, magazines & Manga</option>
          <option value="Fashion">Fashion</option>
          <option value="Collectables">Collectables</option>
          <option value="Electronics">Electronics</option>
          <option value="Garden & Outdoor">Garden & Outdoor</option>
          <option value="Health & Beauty">Health & Beauty</option>
          <option value="Home & Furniture">Home & Furniture</option>
          <option value="Jewellery">Jewellery & Watches</option>
          <option value="Motor Vehicles">Motor Vehicles</option>
          <option value="Music">Music & Audio</option>
          <option value="Sports & Leisure">Sports, Fitness & Leisure</option>
          <option value="Toys & Games">Toys & Games</option>
          <option value="Tools & DIY">Tools & DIY</option>
        </select>

      <div id="subcategory-box" class="subcategory-box"> 
        <label id="subcategory-label" for="subcategories">Item Subcategory:</label>

<select id="subcategories"></select>

</div>

CSS

/* Use first one if you need to use id for the element otherwise for class use second one */

/* #subcategory-box {
  position: relative;
  display: inline-block;
height: 60px;
  bottom: 30px;
  left: 35px;
  display: none;
}
 */
.subcategory-box {
  position: relative;
  display: inline-block;
/*   height: 60px;
  bottom: 30px;
  left: 35px;  */
  display: none;
}

JavaScript

function displaySubCategory() { 
  const category = document.getElementById('item-category');
  
  // Use if you need to check if with id
  //const subcat = document.getElementById('subcategory-box');
  
  // If you need to use class on the element 
  //const subcat = document.getElementsByClassName('subcategory-box')[0];
  
  // Or use this for class
  const subcat = document.querySelector('.subcategory-box');

  subcat.style.display ='inline-block'; 

};