Copy and paste a folder

I am trying to make a copy of an OpenFoam tutorial folder to my desktop through Ubuntu since I don’t want to mess up the original files. I use the command line:
cp -r $FOAM_TUTORIAL /mnt/c/Users/username/Desktop

but this error pops up
cp: missing destination file operand after ‘/c/Users/username/Desktop’

Can I get any help with this?

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Author: user421564