Description states:

returns a list of items without any elements with the same value next to each other and preserving the original order of elements.

Your code fails for cases like [1, 2, 3, 1]. The expected result is [1, 2, 3, 1], but your code outputs [1, 2, 3].

So, conclusion, it doesn't matter that you return a list or not. Returning a list won't make the result invalid. It's the item and its' order.

It doesn't expect a string, actually. It expects Enumerable (as you can see from the solution setup). String and List are both Enumerables. The test has handled it correctly. The reason why in the error it said Test Failed Expected is <System.String>, actual is <System.Collections.Generic.List`1[System.Char]> with 4 elements Values differ at index [4] Missing: < 'A', ... > is because in your enumerable results, the value at certain index is different from the expected enumerable. So, the tests has handled this correctly.

It looks like that happens only in C# version afaik.