Hi, I bought the kartsim pack off of steam and have the 3 tracks but only have the rental kart vehicle not the X30. Is there anything else you need to do to be able to use that kart?
Try unsubscribing/resubscribing to the pack in steam. If you're not sure how to do that, firstly exit completely out of rF2, then in your steam library at the top left you will see your name, hover over it and a dropdown list will appear, select inventory at the bottom. That will show you all your purchased items, click on the KartSim pack and centre right of your screen will be a button to view item in workshop. Select that and hit the unsubscribe button. Fire up rF2 and that should unsubscribe( it has happened to me in the past where doing this doesn't unsubscribe but actually finishes subscribing me to "missing" content) so check. If it was indeed unsubscribed then repeat the process to subscribe to the pack and it should download properly.