You don’t have a have_rows()
loop, so you can’t use get_sub_field()
<?php
$theGroup = get_field('group_field_name');
if( $theGroup ): ?>
<?php if ( $theGroup['truefalse_field_name'] ): ?>
<div class="class_name"><?php echo $theGroup['text_field_name']; ?></div>
<?php else: ?>
<?php endif; ?>
<?php endif; ?>