You have to edit this file in your template, if the file is not exist please copy it from the original theme to your theme.
app/design/frontend/{VendorName}/{ThemeName}/Magento_Catalog/templates/product/view/type/default.phtml
<?php
/**
* Copyright آ© Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
// @codingStandardsIgnoreFile
?>
<?php /* @var $block \Magento\Catalog\Block\Product\View\AbstractView */?>
<?php $_product = $block->getProduct() ?>
<?php if( $_product->getData("not_saleable")) return; ?>
<?php if ($block->displayProductStockStatus()): ?>
<?php if (!$_product->isAvailable()): ?>
<div class="stock unavailable" title="<?= /* @escapeNotVerified */ __('Availability') ?>">
<span><?= /* @escapeNotVerified */ __('Out of stock') ?></span>
</div>
<?php endif; ?>
<?php endif; ?>