I've just installed a fresh install of Ubuntu 17.10 Desktop. I've barely made any changes. Just trying to configure XRDP now according to this description.
Now I'm trying to install an app (Gnome Tweak) from the Ubuntu Software-center. However, this fails. "Unable to install "[app]": you do not have permission to install software.
I've looked for a solution and found this. However, my there's nothing wrong with my org.debian.apt.policy file.
Using sudo in a terminal doesn't give an error.
How to fix this?