You will definitly get this error if your are not make sure of renaming the release.config as web.config..I also enjoyed with this error a lot :) and hope this will help you all to continue your work immediately.
Compiler Error Message: BC30451: Name 'Config' is not declared.Source Error:
Dim AutoUpgrade As Boolean
If Config.GetSetting("AutoUpgrade") Is Nothing Then
AutoUpgrade = True
Source File: D:\Inetpub\wwwroot\intranet\Website\App_Code\Global.asax.vb