Get an error when I call object's properties in Array

The issue is with i <= users.length , it need to be i < users.length.i++ will increment the value of i by 1 but the length is three and the index start from 0 so the elements are occupied till second index, hence i <= users.length will actually try to access the element in the third index which is unefined

let users = [{
    firstName: "John",
    lastName: "Doe",
    age: 34
  },
  {
    firstName: "Jack",
    lastName: "Jackson",
    age: 23
  },
  {
    firstName: "Ann",
    lastName: "Watson",
    age: 24
  }
];

function getUsers(users) {
  for (var i = 0; i < users.length; i++) {
    console.log(users[i].firstName + " is " + users[i].age + " years old!");
  }
}
getUsers(users);

Your for loop exceeds the available array items. The heighest ID is users.length - 1.

So the easiest way is to remove the equal-sign.

Also think of using the buildt in forEach or map function like this:

function getUsers( users ) {
  users.forEach( u => {
    console.log( `${ u.firstName } is ${ u.age } years old!` );
  } );
}

it should be less than length not less than equal to

for (var i = 0; i < users.length; i++) {
   console.log(users[i].firstName + " is " + users[i].age + " years old!");
}

You can use for-of loop

let users = [
{
    firstName: "John",
    lastName: "Doe",
    age: 34
},
{
    firstName: "Jack",
    lastName: "Jackson",
    age: 23
},
{
    firstName: "Ann",
    lastName: "Watson",
    age: 24
}
];
function getUsers(users) {
    for (var user of users) {
         console.log(user.firstName + " is " + user.age + " years old!");
    }
}

getUsers(users);