The program doesn't need to be in any specific folder, however, in saying that, I do remember that some users who had this issue, folder names was one of these issues, example:
You say you have the program in your Downloads folder, so it would look something like below:
C:\Users\Owner\Downloads\Injector
Of course the folder "Owner" above will be different to yours, it will be whatever you set it to when you created your profile login. Now, lets say you have a special character for the user name, like "Pokémon" for example, see below:
C:\Users\Pokémon\Downloads\Injector
I have seen some cases where the injector program didn't like the "é" character, a possible quick fix in this scenario would be to create a new user dummy account on the PC and assign it with Administrator rights, and name it something short with no special characters or spaces. Login to this new dummy account and try the injector program again. Also try to limit the folder name with no spaces where you have your injector program.
Above is one possible fix for your issue, even if you don't have any special characters in your username, I suggest try creating a dummy account anyway and try to inject using that account. If this fails, then it could be .Net framework related issue.