That message doesn’t appear to be directly related to the plugin as has_cap isn’t used by it but maybe by some other plugin you are running.
Are you running the most up to date version of wordpress? Have you tested the plugin on the default theme without any other plugins activated?
It should then hopefully work for you and then you can then turn on the other plugins and your current theme on one by one to work out where the conflict is. I would be very interested to learn what it is conflicting with it so I could provide you a work around.
