For all series, we provide a list of required content on the event page on the website.
Make sure you have installed all of the used DLC content and make sure you own the DLC tracks and the car you're driving.
Find the server-information channel for your series on our Discord. This contains a link to the skin pack we provide. The skin pack is not publicly available and not listed in the public Steam Workshop, so you need the link to subscribe to it. Make sure you subscribe to all content that's listed as required for that skin pack. You'll be prompted to do so when you click subscribe for the skin pack.
rFactor 2 can be temperamental when it comes to the order in which you try to install content. Trying to install an update for content that is not installed yet can block the installation of the main content. The solution for that issue is to unsubscribe from the skin pack only, then start the game to allow everything else to install properly, then subscribe to the skin pack again and restart the game to let that install.