I have had the same problem, it depends on how long you've had it up. Please provide more details about your link such as...

- Is your GamerTag showing in the black box on the Account Linking page in your profile?

- How long have you let it sit and played the game while it showed your GamerTag in the black box?

I have had the same problem, but I just made a whole new GamerTag and it worked immediately and I did call Rockstar Games for this question "Is the Audi R8 like a Downloadable Content File? Can it be used on all GamerTags?" and they said yes so I'd suggest making a new GamerTag and playing the game then.

OR

Call Rockstar Games USA Support - 18664055464

You should get a Rockstar Games Representative to help you with your problem, they will give you steps and tell you what to do to possibly get it to link with the GamerTag you have now... BUT none of this worked for me so I suggest making a new GamerTag and link it right after activating its membership then play the game, no others, just MCLA on that GamerTag, then after it links (it will show the cover of MCLA if it is linked) you can play other games. Just go for the Audi R8 on a separate GamerTag once you open it, it will be available for other files.

If this doesn't help too bad because its what I did. Good Luck,
MCLAMAXPRO out.

You have to link your gamer tag to the RockStar Social club and complete a serise of challenges to get it.

You do as said abouve and yo have to do some challenges before you can unlock the r8

It won't link until you change your gamertag for some reason