Saving custom input from settings sidebar of pages in WordPress

You have forgot to add name of the input box so kindly modify your code for meta box creation as below

function j_meta_box_cb( $post )
{
    $value = get_post_meta( $post->ID, 'j-icon', true );
    
    ?>
   
  <input id="j-meta-box" type="text" name="j-icon"value="<?php echo esc_attr( $value ) ?>"></input>
<?php
} 

Let me know still if you hv any queries.