Actually there are a lot more skins that you can't add (Bentley #1 I think, couple of McLaren 650s to name just a few). I contacted Chris Elliot about it, so he told me that he will try to repro it. Actually, the fix is pretty simple, it just needs an update to Vehicle description so that each description is unique (without a, or b or c as number) and each livery must have two digit number so for ex.. #1 should be #01 since they have some strange logic in how they select car from clicked livery. So this should be a quick fix....just different (unique) string for description...and I offered to provide this descriptions for each unselectable vehicle....but I'm not optimistic about how quickly (if) this will be fixed....this is a long standing bug
you can fix it for yourself if you don't want to wait for official fix. But fix involves repacking that car, and repacking again with each version change, which I was doing before, but got tired of it since its payed content and I expect that devs fix payed content bugs, especially simple as this one. PM me if you have the will to do it yourself and need help in how.
do you know how to delete skins for official content, I don't want all the skins for the Mclaren, or others, each time I add the 720S in a race, there are only Mclaren 720S cause there are too much skins
"Easiest would be to manually pick AI cars once on the track, that way you can select what ever cars you want (apart from the bug described in opening post). Longer version, extract rfcmp for mclaren, remove liveries that you don't want/need and repack it back. Only problem is that when S397 updates that with a newer version, you'll need to do that again (whole repacking thing).