Yes, trying to download a demo if there is something in your cart and you have no funds is impossible you should be able to download a demo no matter whats in your cart. I didn't realize that having a game in my cart was keeping me from downloading a demo. You shouldnt have to delete items in your cart to download a demo
you have no funds in your wallet and there is a game sitting in your cart you click try a demo the demo goes to your cart, now you have a demo and a game in your cart from whenever, you should be able to click the demo in your cart and proceed to download and play it regardless of the game in your cart.
I have said it before but that is the reason why I couldn't proceed to play the demo because there was a game in my cart that should be fixed and I had no clue as to why I couldn't proceed with the demo.