Override vendor/magento/module-sales/view/frontend/templates/email/items/order/default.phtml
to your theme
app/design/frontend/vendor/theme/Magento_Sales/templates/email/items/order/default.phtml
Now add sku <td class="item-sku"><?= $block->escapeHtml($block->getSku($_item)) ?></td>
where you want to show sku
I added sku after product name here is final default.phtml
<?php
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
// @codingStandardsIgnoreFile
/** @var $block \Magento\Sales\Block\Order\Email\Items\DefaultItems */
/** @var $_item \Magento\Sales\Model\Order\Item */
$_item = $block->getItem();
$_order = $_item->getOrder();
?>
<tr>
<td class="item-info<?php if ($block->getItemOptions()): ?> has-extra<?php endif; ?>">
<p class="product-name"><?= $block->escapeHtml($_item->getName()) ?></p>
<?php if ($block->getItemOptions()): ?>
<dl class="item-options">
<?php foreach ($block->getItemOptions() as $option): ?>
<dt><strong><em><?= /* @escapeNotVerified */ $option['label'] ?></em></strong></dt>
<dd>
<?= /* @escapeNotVerified */ nl2br($option['value']) ?>
</dd>
<?php endforeach; ?>
</dl>
<?php endif; ?>
<?php $addInfoBlock = $block->getProductAdditionalInformationBlock(); ?>
<?php if ($addInfoBlock) :?>
<?= $addInfoBlock->setItem($_item)->toHtml() ?>
<?php endif; ?>
<?= $block->escapeHtml($_item->getDescription()) ?>
</td>
<!-- Here add sku column -->
<td class="item-sku"><?= $block->escapeHtml($block->getSku($_item)) ?></td>
<td class="item-qty"><?= /* @escapeNotVerified */ $_item->getQtyOrdered() * 1 ?></td>
<td class="item-price">
<?= /* @escapeNotVerified */ $block->getItemPrice($_item) ?>
</td>
</tr>
<?php if ($_item->getGiftMessageId() && $_giftMessage = $this->helper('Magento\GiftMessage\Helper\Message')->getGiftMessage($_item->getGiftMessageId())): ?>
<tr>
<td colspan="3" class="item-extra">
<table class="message-gift">
<tr>
<td>
<h3><?= /* @escapeNotVerified */ __('Gift Message') ?></h3>
<strong><?= /* @escapeNotVerified */ __('From:') ?></strong> <?= $block->escapeHtml($_giftMessage->getSender()) ?>
<br /><strong><?= /* @escapeNotVerified */ __('To:') ?></strong> <?= $block->escapeHtml($_giftMessage->getRecipient()) ?>
<br /><strong><?= /* @escapeNotVerified */ __('Message:') ?></strong>
<br /><?= $block->escapeHtml($_giftMessage->getMessage()) ?>
</td>
</tr>
</table>
</td>
</tr>
<?php endif; ?>
Also you need to override vendor/magento/module-sales/view/frontend/templates/email/items.phtml
for add new for sku
app/design/frontend/vendor/theme/Magento_Sales/templates/email/items.phtml
<th class="item-qty">
<?= /* @escapeNotVerified */ __('Sku') ?>
</th>
OUTPUT:
![enter image description here](https://i.sstatic.net/WDd8b.png)