I've just taken a quick look and this is what I've found.
Before you begin, the 2018 season must be selected. Once that's done, here is where I could select the cup car:
Endurance 24H Race Weekend
Custom Race Weekend
Where I could not select the 911 cup car:
Endurance 3H Race Weekend
Endurance 6H Race Weekend
Sprint Race Weekend
Thanks @EsxPaul, I’ll recheck tom. Wait, just to confirm, where exactly do I select 2018 (some screenshot pls)? Might be a different screen from where I thought I should... I always changed it when I’m inside screen where you choose track/team/car. Thanks.
Click the game mode box, select one of the modes (In my post above) that allow the cup car to be used then confirm:
Once you have selected the relevant game mode and then confirmed it, you'll be brought back to the above screen.
Now you can click the Team box on the right of the screen and the cup car will be available.