Add category descriptions to category pages

Unfortunately my theme doesn’t support category descriptions. I have carried out a lot of research to try and find out how to implement these descriptions on page. Most of the forums say to add this piece of code to the category.php or archive.php file

<?php echo category_description( $category_id ); ?>

However, it is not working for me. Does anyone have a solution for this? would really appreciate any help.

Thank you