Do you have an actor name that is empty?
Viewing post in [CGMZ] Save File for RPG Maker MZ comments
It is drawing the actor name on their face image, but the actor's name is undefined so when it goes to call string functions on the actor name it crashes because undefined is not a string. You can try copying the below JS into the bottom of the plugin file and it should start working:
CGMZ_Window_SaveFileDisplay.prototype.onFaceLoaded = function(args) {
this.drawFace(args.name, args.index, args.x, args.y, args.width);
const y = args.y + ImageManager.faceHeight - this.lineHeight();
this.contents.fontBold = true;
this.contents.outlineWidth = 6;
if(args.actorName) {
this.CGMZ_drawTextLine(args.actorName, args.x, args.y, args.width, "center");
this.CGMZ_drawTextLine(CGMZ.SaveFile.LevelText + args.actorLevel, args.x, y, args.width, "right");
}
this.contents.fontBold = false;
this.contents.outlineWidth = 3;
};