I just solved this problem with shopify customer service team. Share here for others if you face the same problem. It happens because the liquid code of gift card in your theme is not completed. I paste my code here in case someone need it:
{% layout none %}
{% assign formatted_initial_value = gift_card.initial_value | money_without_trailing_zeros %}
{% assign formatted_initial_value_stripped = formatted_initial_value | strip_html %}
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="format-detection" content="telephone=no">
<title>Here's your {{ formatted_initial_value_stripped }} gift card for {{ shop.name }}!</title>
<meta name="description" content="Here's your gift card!">
{{ 'gift-card.css' | shopify_asset_url | stylesheet_tag }}
{{ 'modernizr.gift-card.js' | shopify_asset_url | script_tag }}
{{ 'vendor/qrcode.js' | shopify_asset_url | script_tag }}
<link href="//fonts.googleapis.com/css?family=Montserrat:400,700" rel="stylesheet" type="text/css">
<!--[if lt IE 9]>
{{ 'vendor/html5shiv.js' | shopify_asset_url | script_tag }}
<![endif]-->
<script type='text/javascript'>
function selectText(element) {
var doc = document;
var text = doc.getElementById(element);
if (doc.body.createTextRange) { // ms
var range = doc.body.createTextRange();
range.moveToElementText(text);
range.select();
} else if (window.getSelection) { // moz, opera, webkit
var selection = window.getSelection();
var range = doc.createRange();
range.selectNodeContents(text);
selection.removeAllRanges();
selection.addRange(range);
}
}
</script>
</head>
<body>
<div class="wrap">
<header role="banner" id="header">
<a href="{{ shop.url }}" class="logo">{{ shop.name }}</a>
<a href="{{ shop.url }}" class="shop-url">{{ shop.url | remove: 'http://' }}</a>
</header>
<main role="main">
<div id="gift-card-outer-container" {% if gift_card.expired or gift_card.enabled != true %}class="disabled"{% endif %}>
<div id="gift-card-inner-container">
<header id="gift-card-header">
<h2>Here's your gift card!</h2>
{% unless gift_card.enabled %}
<span class="tag">Disabled</span>
{% endunless %}
{% if gift_card.expired and gift_card.enabled %}
<span class="tag">Expired on {{ gift_card.expires_on | date: "%d/%m/%y" }}</span>
{% endif %}
{% if gift_card.expired != true and gift_card.expires_on and gift_card.enabled %}
<span class="tag light">Expires on {{ gift_card.expires_on | date: "%d/%m/%y" }}</span>
{% endif %}
</header>
<div id="gift-card-holder">
<div class="corner top-left"></div>
<div class="corner bottom-right"></div>
<div class="corner top-right"></div>
<div class="corner bottom-left"></div>
<div id="gift-card">
<img src="{{ 'gift-card/card.jpg' | shopify_asset_url }}" alt="Gift card illustration">
{% assign initial_value_size = formatted_initial_value | size %}
<div id="gift-card-amount" class="{% if initial_value_size > 6 %}medium{% endif %}">
{% if gift_card.balance != gift_card.initial_value %}
<span class="tooltip-container"><span class="tooltip-label">{{ gift_card.balance | money }} <small>left</small></span><span class="tooltip-triangle"></span></span>
{% endif %}
<strong>{{ formatted_initial_value }}</strong>
</div>
{% assign code_size = gift_card.code | format_code | size %}
<div id="gift-card-code-outer" class="{% if code_size <= 25 %}large{% elsif code_size > 25 and code_size <= 30 %}medium{% else %}small{% endif %}" onclick="selectText('gift-card-code-digits');">
<div id="gift-card-code-inner">
<strong id="gift-card-code-digits">{{ gift_card.code | format_code }}</strong>
</div>
</div>
</div>
</div>
<div id="gift-card-instructions">
<p>Use this code at checkout to redeem your gift card.</p>
</div>
<div id="qr-code"></div>
<script>
new QRCode(document.getElementById("qr-code"), {
text: "{{ gift_card.qr_identifier }}",
width: 120,
height: 120
});
</script>
<div id="gift-card-actions">
<a href="{{ shop.url }}" class="btn center" target="_blank">Start Shopping</a>
<a href="#" class="action-link left" onclick="window.print();"><i class="ico-16 print"></i>Print</a>
</div>
</div>
</div>
</main> <!-- / Main -->
<footer role="contentinfo">
{% if gift_card.pass_url %}
<a href="{{ gift_card.pass_url }}"><img id="apple-wallet-badge" src="{{ 'gift-card/add-to-apple-wallet.svg' | shopify_asset_url }}" width="120" height="40" alt="Add To Apple Wallet"></a>
{% endif %}
</footer>
</div>
</body>
</html>
