[munkyeetr]

Right-click on the file and choose Properties or in a Terminal window open to the directory where the file is and type:

    ls -la

Does it list you as the Owner of the file? I have no problem running that command on a file that I've created.

Also, as I said before, sudo will not work if you're not listed in the the /etc/sudoers file.

What Linux distribution are you running?