you are asked for this: https://en.wikipedia.org/wiki/Complement_(set_theory)#Relative_complement

you are doing this: https://en.wikipedia.org/wiki/Symmetric_difference

i am close to giving up at this point, filtered the 2 arrays to get the unique values for both and the returned the concatenated unique values as an array, i even passed the sample arrays and got the exact same answers, but it's still not accepting my answer

Not sure what your question is, but seems like you've solved it, so closing...

Missing using System.Linq perhaps...

why i can't use a.ToList()?

Just array. You are overthinking it.

im beginner, im not understand this tester want me return. is it need to return a string or array ?.
the logic is okay but then still says not correct.

This seems 8th kyu easy. Am.... I.... EVOLVING?!?!?

The assertion messages display the inputs clearly, actual and expected results clearly, closing.

This suggestion cannot be implemented under the constraints of different programming language's paradigm. Also, the only thing you can restrict when it comes to lifting up the challenge to the next level is probably code length, which fits well in a code-golf task, but not here, so closing.

Not a kata suggestion. OP solved it, closing...

I made it hard for myself, i filter a[] and b[] for any unique numbers via two sets. (Maybe a new kata?)
Who can read is clearly in the advantage :D
TLDR: Read instructions properly and someone please make the test message less ambigous :)