YEP Skill Core is no longer available in MZ. I believe it is part of the VisuStella plugin suite now which unfortunately is not compatible with Card Game Combat (as per their request). An alternative plugin for MZ would be Eli's Skill Costs which will allow you to specify other attributes as the costs for your skills. While this plugin is not currently compatible with CGC now, we can offer to extend compatibility to it as part of the upcoming Version 1.5.0 patch (which should be out by end of January).