I'm glad someone finally asked.
The page must have at least 2 paragraphs and the first paragraph must be at least 270 chars long it seems. I just did rigorous testing and discovered this.
the below will work
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="Description" CONTENT="Sample for Google assistant">
<title>Sample for Google assistant</title>
</head>
<body>
<h1>A title</h1>
<p>
Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.
</p>
<p>
Here is an text sample.
</p>
</body>
</html>
In addition even though you didn't ask this: Google assist also seems to not be able to read content that is more than 4 divs deep.
For example I had to comment out the below divs to get my own webpage to work, not exactly sure why but i assume its the div depth.
<html lang="en">
<body class="bg-light">
<div class=" ">
<div class="content">
<div class="container container-small bg-white Pa_50px rounded my-2">
<div style="" class="row">
<!-- <div style="" class="Wi_100Per BoSi_BoBo"> -->
<!-- <div style="vertical-align:top;" class="Wi_100Per d-inline-block"> -->
<!-- <div class="p-2"> -->
<!-- <pre style="white-space: pre-wrap;"> -->
<p>
Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.Here is an text sample.
</p>
<p>
Here is an text sample.
</p>
<!-- </pre> -->
<!-- </div> -->
<!-- </div> -->
<!-- </div> -->
</div>
</div>
</div>
</div>
</body>
</html>
Footnote: of course make sure you're using Chrome to start with. Also I acknowledge that you experienced that you only need one paragraph.