Error Accessing of c#.net Web Application

I have created C#.net Web Application And Added Few html pages from Existing Html Pages. from that added html pages try to access the asp C#.Net Pages. at that time i Get An error
<customErrors mode="Off"/>
<customErrors mode="RemoteOnly"/>

After uploading this application on the server i have to access some Html Pages outside from the Application. So I should convert or add all html pages to that application