After upgrading Visual Studio or .NET framework, app.config or web.config file may generate the message "Could not find schema information for the element..." upon building the project. This means the schema needs to be upgraded too. Follow the steps below to resolve the issue.

Create a new schema based on app.config. open app.config file, from menu XML>Create Schema or toolbar button "Create Schema", Visual Studio will create a xsd schema.

Check the "Properties" tab of the app.config where there is a property named Schemas. Find out the path of the current schema file. Save the new schema in an appropriate place. Change the property Schemas to the new schema path. Click on the edit button "..." or menu XML>Schemas to show XML Schema dialog. You will find both the original dotnetconfig schema and your own newly created one are selected. Note that you can use multiple schema files by selecting "Use this schema" in Column "Use" in XML Schema dialog.