I've validated my html page and included the code below for advertising. It happens that the advertisements do not appear. I tested it on my cell phone and it does not show up. And neither on the PC does not appear. I waited for the time needed etc.
<amp-ad
layout="fixed-height"
height="250"
type="adsense"
data-ad-client="ca-pub-xxxxxxxxxxx"
data-ad-slot="xxxxxxxxxxxx">
</amp-ad>
<head>
<script async custom-element="amp-ad"
src="https://cdn.ampproject.org/v0/amp-ad-0.1.js"></script>
</head>