I was facing the same issue, but could fix it with the hint of Venkateshkoduri.
The steps are:
1. Open the Power Apps Portal Management
2. Navigate to the menu 'web templates'
3. In the list of web templates, filter the website to make sure you just see the webtemplates of the website where you are facing the issue
4. Find the web template named 'Default Studio template'. and open it. In my case, the code was empty for some reason.
5. Add the following code snippet:
<!-- Default studio template. Please do not modify -->
<div id="mainContent" class = "wrapper-body" role="main">
{% include 'Page Copy' %}
</div>
