I see, thanks!

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].

I personally would name the variables not after you and your opponent but after attacker and defender. It's less ambiguously that way. (My opponent could be my attacker or he could be the one I'm attacking.) Also a nitpick but the 50 is a bit arbitrary and I don't see why that couldn't have been an argument as well.

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.