Display Error when format date in Laravel

I am beginner in Laravel.

I have date:

Illuminate\Support\Carbon @1577636272 {#912 ▼
  date: 2019-12-29 17:17:52.0 Europe/Warsaw (+01:00)
}
$userLastActivity = $userLastActivity->format('Y-m-d H:i');

When I try convert it I have error:

Call to a member function format() on null

How can I repair it?

via Chebli Mohamed