That was the same problem I was having at first. I did some reading, and you can't download Itch games to the program files folder. Are you trying to install Itch there, or just the games? I'm not sure if it's an option to install the Itch app to the D: drive. But if you can get the Itch app onto your C: drive (which isn't that big of a file), then you can install all of your games onto the D drive. What I did, I made folders that match the same heiarchy that Itch builds on your C: drive, the only difference is that on your D: drive, there won't be a "/user/" folder. Once you do that, in Itch, when you Install the game, find the folder you just built to put it in, and select it, and it will install. Does this make sense?