FM7 and these forzathon challenges are very buggy. If you pick the right car and event you can negotiate through them. I did and got all 3 rewards. This YouTube video helped.
But, the Ford Fiesta Autocross didn't work for me. The Ford GT one did, which I did 3 times in a row. As for the Porsche race, I did a 30 sec. Timed race in one Porsche and got the pop-up that the challenge was completed, but it did NOT get checked on the line of Forzathon events and I did NOT get the Astronaut Driver Gear. I redid the 30 sec. Timed race using the Porsche 928 and it all came up roses.
Refer to the "This is Forza Motorsport 7" challenge for other comments.