Unable to preview page - Permission to the requested document was denied

Question

This is a fresh 9.3 instance. I'm unable to preview any page. On clicking preview, it opens a page which has this error:

I'm logged in as Admin. It did not happen with other instances like 8.2. Are there any settings to be made.

Here is my MySiteDefinition.config:

<site name="MySite" virtualFolder="/" physicalFolder="/" rootPath="/sitecore/content/MySite" 
startItem="/home" language="ja-JP" database="web" domain="mysite" allowDebug="true" 
cacheHtml="true" htmlCacheSize="50MB" enablePreview="true" enableWebEdit="true" 
enableDebugger="true" disableClientData="false" 
dictionaryPath="/sitecore/content/MySite/Global/Dictionary" dictionaryAutoCreate="false" />

Answer 1

Check the below setting in Sitecore.config

<!-- PREVIEW AS ANONYMOUS
       This setting specifies whether Sitecore will use the Anonymous user account when you preview an item.
       If true, Sitecore uses the Anonymous user account when you preview an item.
       If false, Sitecore uses the current user account when you preview an item.
       Default value: true
  -->
    <setting name="Preview.AsAnonymous" value="false" />

Default is "true" so whenever you preview in Sitecore versions 9.1+, it will take "Anonymous" user context and if you have security to disable access for anonymous then the page might throw an error. Change it to false if you want to run preview in logged-in user context.

In your case the error screen shot clearly says that the preview user was "extranet\Anonymous" though you logged in as admin and previewed.