Guess what... some facts:
In those files:
components\com_j2store\templates\default\view_tabs.php
components\com_j2store\templates\bootstrap3\view_tabs.php
all tabs in product detail view are defined
and there is code:
<?php if($this->params->get('item_show_product_specification')):?>
<li class="<?php echo isset($set_specification_active) && $set_specification_active ? 'active' : '';?>" >
<a href="#specs" data-toggle="tab"><?php echo JText::_('J2STORE_PRODUCT_SPECIFICATIONS')?></a>
</li>
<?php endif;?>
Which check if specification is active - if is not - does not display it.
So you have to read J2Store manual as I mentioned before.
Because this part of code was not:
a) developed by us
b) overridden by us, too
Get lucky , be real superman.