I'd contact Codemasters customer service, they told me that: "Please ensure that you have installed the latest patch for the game
before installing the Drag Pack DLC. If you are unsure when you
installed the latest patch, please uninstall the DLC, make sure that you
are connected to the internet and signed in when you start the game, so
that it checks for any patches. Once you are sure you have the latest game patch, you should be able to install the DLC again." And it works!