You need to use cmuser with `Ttm-AddWebsite`, as noted in [this answer from Rick][1].

If this doesn't help, it's likely that you haven't registered the WebCapability. So check that it's correctly set up in the `cd_storage_conf.xml` (probably of your discovery service) and that you have run the update: 

    java -jar discovery-registration.jar update


  [1]: http://tridion.stackexchange.com/a/14388/129