error page "Server Error in '/' Application

I'm still seeing "Can't reach this page" on all my site collections and this screen when trying to create a new site.

Server Error in '/' Application.

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Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed.

Details: To enable the details of this specific error message to be viewable on the local server machine, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "RemoteOnly". To enable the details to be viewable on remote machines, please set "mode" to "Off".

<!-- Web.Config Configuration File -->

<configuration>
<system.web>
<customErrors mode="RemoteOnly"/>
</system.web>
</configuration>

Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.

<!-- Web.Config Configuration File -->

<configuration>
<system.web>
<customErrors mode="On" defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>

Please open "web.config" of the web application your site collections host on and modify the <customErrors> tag using above method in my answer, you will see detailed information then.

That didn't work for me

Changing "mode" attribute of tag <customErrors> to "RemoteOnly" won't fix this issue but we can find detailed error messages about this issue. Please make sure you have correctly set the attribute by the method I mentioned in this post. You can share the screenshot or the web.config file of the web applciation here so that we can verify it for you. For this kind of error, we must first obtain detailed error information before troubleshooting.

Here is a snippet of the web.config file below

<RuntimeFilter Assembly="Microsoft.Office.Server, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Class="Microsoft.Office.Server.Audience.AudienceManager" BuilderURL="audience_chooser.aspx" />  

</SharePoint>
<system.web>
<httpHandlers />
<customErrors mode="RemoteOnly" />
<httpRuntime maxRequestLength="256000" maxUrlLength="401" executionTimeout="1500" requestValidationMode="2.0" requestPathInvalidCharacters="\,,?,<,>" />
<authentication mode="Forms">
<forms loginUrl="/_login/default.aspx" />
</authentication>
<identity impersonate="true" />
<authorization>
<allow users="" />
</authorization>
<httpModules>
<add name="FederatedAuthentication" type="Microsoft.SharePoint.IdentityModel.SPFederationAuthenticationModule, Microsoft.SharePoint.IdentityModel, Version=16.0.0.0, C