if you use function expression for that method you have to remember about hoisting

It won't work with 'class' syntax for some reason.
You have to use the approach shown here: https://www.youtube.com/watch?v=gdjfsDlkH7A

Can someone tell me what the error is?

Uses Object.defineProperty to get and set instance weight
Value is not what was expected

Add one more test - set felix.weight second time and check average. This will surely show some broken solutions.

Use Object.defineProperty to write custom setters and getters

This might have been the (only solution?) norm when this kata was created, but now that JavaScript has a proper class syntax with getters and setters, forbidding the use of modern language features is ridiculous.

If anybody's interested:

Test.it("should have an averageWeight method", function () {
  Test.assertEquals(typeof Cat.averageWeight, 'function');
});

Nobody's going to change it now.

SyntaxError: Identifier 'Cat' has already been declared

Why is there a Cat hidden in preloaded?

I've imlement this method (all test are passed). What does this test check? method typeof?? If yes, my code also passed sample tests, which are check method typeof too

seems pretty clear, doesn't it? :o
you have to define that averageWeight method, where appropriate for the current task.