Visual-studio – Unable to find the requested .Net Framework Data Provider. It may not be installed. (System.Data.SQLClient)

I have seen reports of people having and additional, self terminating node in the machine.config file. Removing it resolved their issue. machine.config is found in \Windows\Microsoft.net\Framework\vXXXX\Config. You could have a multitude of config files based on how many versions of the framework are installed, including 32 and 64 bit variants.

<system.data>
    <DbProviderFactories>
        <add name="Odbc Data Provider" invariant="System.Data.Odbc" ... />
        <add name="OleDb Data Provider" invariant="System.Data.OleDb" ... />
        <add name="OracleClient Data Provider" invariant="System.Data ... />
        <add name="SqlClient Data Provider" invariant="System.Data ... />
        <add name="IBM DB2 for i .NET Provider" invariant="IBM.Data ... />
        <add name="Microsoft SQL Server Compact Data Provider" ... />     
    </DbProviderFactories>

    <DbProviderFactories/>  //remove this one!
</system.data>

From my experience, that error means that the value of the providerName attribute on the connection string in your web.config is either incorrect, or the provider literally is not installed. If your providerName is set to System.Data.SqlServerCe.4.0 (SQL Server Compact), which is not uncommon in development, I can guarantee you that it's not installed on your web host; it's only used inside Visual Studio for development. You probably just need to change it to a the real SQL Server provider: System.Data.SqlClient.