any one can explain why it's appear

Server Error in '/' Application.
---------------------------------
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 remotely (for security reasons). It could,
however, be viewed by browsers running on the local server machine.

Details: To enable the details of this specific error message to be viewable on
remote machines, 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 "Off".


<!-- Web.Config Configuration File --><configuration> <system.web>
<customErrors mode="Off"/> </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="RemoteOnly" defaultRedirect="mycustompage.htm"/>
</system.web></configuration>