For those who have difficulty in logging into that webservice account or if like me doing so would cause other operational challenges to an existing site then note that you could always just update the policyagreed column to 1 on the mdl_user table for the webservice id.
I wouldn't suggest doing this as a standard process but in a pinch where you are tying to enable a site policy on an existing site then this will also resolve the webservice issue described above