The message tells you what to to:
"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 "Off"."
You will then be able to see the details about the error. Without them, neither you nor I can do anything but guess.
Here is an example from one of mine:
<configuration>
<appSettings file="WebAppSettings.config">
...
</appSettings>
<connectionStrings configSource="WebConnectionString.config"/>
<system.web>
<authorization>
...
</authorization>
<customErrors defaultRedirect="~/Whoops.aspx" mode="Off" redirectMode="ResponseRewrite">
<error statusCode="403" redirect="~/NoAccess.aspx"/>
...
</customErrors>