I had same problem when I tried to upgrade using the Safari on iPad. I would suggest try to upgrade on a different tool (phone, computer, iPad, etc). Once I have paid for the course, I had no problem accessing the materials on iPad, iPhone, or Mac.
All of his LR lessons use questions from before PT36 as practice with explanation videos, and there are more practice sets with each package upgrade... so if you're looking for some, thats where you can find them :D