How to change folder access if it says "You have unknown access"?
I have encountered this problem before, on the Downloads folder, and on other folders. Removing the directory and it's contents, and recreating the directory worked for me.

The commands below are to be entered in the Terminal

Copy content before deleting:

mkdir ~/Temp
sudo cp -R  ~/Downloads ~/Temp/
sudo chmod -R 755  ~/Temp 


To remove your damaged Downloads folder:

rm -rf ~/Downloads, or if that doesn't work, sudo rm -rf ~/Downloads

To recreate a new empty one after removal:

mkdir ~/Downloads; chown $USER ~/Downloads