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, ''). The '{0}' attribute currently references a version that is later than the installed version of the .NET Framework. Specify a valid target version of the .NET Framework, or install the required version of the .NET Framework.