This Code worked for me
while ( odrin_have_rows('read_the_book_chapters', $post_id) ) : the_row(); if (odrin_get_sub_field('sub')){
$output .= '<li><ul>'; }
$output .= '<li><a href="#item'. $item_num = 1 . '">' . odrin_get_sub_field('title', $post_id) .'</a></li>';if (odrin_get_sub_field('sub')){
$output .= '</ul></li>';}$item_num++; endwhile;
I was missing <li>
of sub menu. Meaning that <ul>
of sub menu should be within <li>
of parent.
Here is the structure of sub menu
<ul>
<li>Parent Item
<ul><li>Sub Menu Item</li></ul>
</li>
<li>Another Parent Item</li>
</ul>