Category cloud can be added by using the taxonomy argument to cause a cloud of categories to display. You can either modify wp-content/yourtheme/single.php from ssh or using ftp OR you can login to wordpress dashboard, and go to “Appearance -> Editor -> ad click “single.php”
Post below line,
<?php wp_tag_cloud( array( 'taxonomy' => 'category' ) ); ?>
After,
<div class="thecontent">
<?php the_content(); ?>
</div>
So, the final change could look like as below,
<div class="thecontent">
<?php the_content(); ?>
</div>
<?php wp_tag_cloud( array( 'taxonomy' => 'category' ) ); ?>
To update into single page, modify the page.php as above.
Reference – https://codex.wordpress.org/Function_Reference/wp_tag_cloud#Display_a_Category_Cloud
