Earlier, I once had a similar problem when I tried to run the GT All-Stars race at the High Speed Ring.
When I clicked on the start button, the game said "Race information invalid" "This race cannot be started", and the game wouldn't let me start the race. My car was not on the starting grid, and the grid was not fully loaded (the grid only had five cars if I remember correctly).
My guess is that the game had some kind of glitch that prevented it from loading in my car plus a few of the AI cars. I don't know why. I think its possible that my game was just having problems reading my GT6 disc (maybe the disc was getting dusty).
To fix my problem, I exited out of GT6 and shut off my PS3 console, and then when I re-started everything, I was able to enter the GT All-Star race and could continue.
Good luck!
GTsail