I don't know C# but looking at the the error message System.InvalidOperationException : Sequence contains no elements I'm guessing what's happening is you are failing on the 5th test case which is testing your code with an input of an empty array - as mentioned in description:

"If you get an empty array/list, return an empty array/list."

So you have to handle that case gracefully, however you implement those kind of situations in C# !

You have to remember that there are dozens of different languages in use on Codewars, and of course some features/tasks are considerably easier/harder than others depending on language - there are some 4-5 kyu katas that are trivial with some imported modules in Python for example (and no, I'm not telling you which ones they are :P)

FWIW, this seems to have approx same number of solves in Python and Javascript (20,000 or so) which suggests it might originally have been a Python authored-kata; if so it doesn't seem too difficult for 8 kyu.

That's a problem with your code, not a kata issue, it's failing the second test.

// first test
1
2   //input
3
4
5

2
3   //output
4
5


// second test
5
3
2   //input
1
4

2
3   //output
4   
5

Read this: https://docs.codewars.com/training/troubleshooting/#kata-bug and post your code.

Fixed that, check your code and fix it.

Because actual and expected are flipped in random tests, your code is generating those nulls.

There are no expected "null" results or inputs, just arrays. What's obvious is that there's some problem with your code, not tests. Please see https://docs.codewars.com/training/troubleshooting for some tips or post your code here (or on discord) using the format shown here: https://docs.codewars.com/references/markdown/#code-block

Agreed