When i run my solution asserting goes right. But "Check" fails.

Your result:[4,4,4,4,6,6,2,2,2]
Right result:[4,4,4,4,2,2,2,6,6]
Fail:frequency_sort([4,6,2,2,2,6,4,4,4])

It says in the mission description:

If two elements have the same frequency, they should end up in the same order as the first appearance in the iterable.

So the "Right result" is wrong.