fair chance you did not import the "System" namespace (line1)

No problem at all.

Is it OK with you if I put the spoiler back? Before I do this you should be able to copy and paste the contents of my reply if you think it will be helpful.

By the way, a good way to get help is to go to the help-solve channel on Discord: ⁠https://discord.com/invite/7U9t33jrgG

I didn't realize. OK I will take away the spoiler warning for a while. But then I will put it back.

You can't un-flag - meaning you can't turn the spoiler warning off. But you still should be able to click on it - not?

I marked it as a Spoiler, because it gives some information to help solve the problem, so some people don't want to see it. But you should be able to click on it and see what I suggested.

It does not look like this to me. Are you sure? Does it get fixed when you click RESET?

Submission tests show you what was the failing array:

Test Failed
  Incorrect answer for array=[]
Assert.That(Calculator.LastDigit(allCases[i].test), Is.EqualTo(allCases[i].expect))
  Expected: 1
  But was:  0

Example tests do not show the array, but you can see the code of the exampel tests, you can modify it at will, so you have everything what's needed to figure out which assertion is failing.

Your code using Math.Pow has no chance to work. It will lose precision very quickly and give inaccurate result. Also num.ToString() called on a large double will not work in an expected way.

You need a uch better idea, and for sure one which does not use floatign point numbers.

You can always see what test goes wrong by doing Console.WriteLine(arr).

Perhaps some reading might help, though this kata does not require to handle default values, only handling of empty list as an edge case is needed.

This would make the task more specific, which is not necessary.

The actual colors tested are irrelevant to solving the kata.

Closing...