The '{0}' attribute in the element of the Web.config file is used only to target version 4.0 and later of the .NET Framework ...

The '{0}' attribute in the  element of the Web.config file is used only to target version 4.0 and later of the .NET Framework (for example, ''). If you are compiling this Web application for a version of the .NET Framework earlier than 4.0, remove the '{0}' attribute from the  element of the Web.config file.