Windows has some weird "folders" like "Desktop", "Favorites" and a few others, that actually do not exists as real "folders" on your hard drive but are aliases, that point to some completely different folder. For example I don't actually have any "C:\Desktop" path, but only a "C:\User\User\Desktop". My current guess would be, that you give the Pack Toolkit such an alias name, but it (currently) cannot properly resolve this alias and takes it as an actual path (which of course does not exist).
So until I get around to fix this bug, I suggest trying to save the pack into some folder that does not contain "Desktop", "Favorites", "User" or "Libraries" in it (and I hope I got the correct English terms for the German folder names I see
).