whmcsCrosterTemplates/store/not-found.tpl

<div class="container">
    <div class="error-container boxed">
		<div class="alert-lg no-data no-border" style="margin: 60px 0 15px;">
			<div class="icon">
				<i class="fas fa-exclamation-triangle"></i>
			</div>
			<div class="text">
				<h3>{lang key='store.addon.notFound.productUnavailable'}</h3>
				<p>{lang key='store.addon.notFound.productUnavailableText'}</p>
				<p><em>{lang key='store.addon.notFound.ref'}: {$productName}</em></p>
			</div>
			<div class="btn-group d-none d-sm-block">
				<a href="{$systemurl}" class="btn btn-primary">
					{lang key="errorPage.404.home"}
				</a>
				<a href="{$systemurl}contact.php" class="btn btn-default">
					{lang key='store.addon.notFound.contactSupport'}
				</a>
			</div>
			<div class="d-block d-sm-none">
				<a href="{$systemurl}" class="btn btn-primary">
					{lang key='store.addon.notFound.contactSupport'}
				</a>
			</div>
		</div>
    </div>
</div>