Hi,

Please try below:

var UserCriteriaValidator = Class.create();
UserCriteriaValidator.prototype = Object.extendsObject(AbstractAjaxProcessor, {

checkUserMatches: function () {
      var userID = this.getParameter('sysparm_userID');
      var userCriterias = [];
userCriterias.push(this.getParameter('sysparm_userCriterias'));
      // Check if the user matches the given criteria
      var check = new sn_uc.UserCriteriaLoader.userMatches(userID, userCriterias);

      // Return the result
      return check;
   },

   type: 'UserCriteriaValidator'
});

Second parameter 'userCriterias' is not string, it should be of type array/list.