[ixth]

Думаю, проблема где-то в отступах и else'ах.

Вот этот вариант работает:

var ATM = {
        is_auth: false,
        current_user: false,
        cash: 1000,
        users:[
            {number: "0025", pin: "132", debet: 675, group: "user"},
            {number: "0001", pin: "000", debet: 1000000, group: "user"},
            {number: "1111", pin: "111", debet: 0, group: "admin"}
            ],
        getcash: function(amount) {
            if (this.is_auth) {
                if(amount > 0 && amount <= this.cash && this.users[this.current_user].debet >= amount){
                    this.cash -= amount;
                    this.users[this.current_user].debet -= amount;
                    return amount;
                } else {
                    return "no money";
                }
            } else {
                return "go auth";
            }
        },
        auth: function(number, pin){
            for (i=0; i < this.users.length; i++) {
                if (this.users[i].number == number && this.users[i].pin == pin) {
                    this.is_auth = true;
                    this.current_user = i;
                    break;
                }
            }
        },
        loadcash: function (bablo) {
            if (this.is_auth) {
                if (bablo > 0 && this.users[i].number == number && this.users[i].pin == pin && this.users[i].group == "admin") {
                    this.cash = this.cash + bablo;
                } else {
                    return "you are not admin";
                }
            } else {
                return "go auth";
            }
        }
    };