Assert.AreEqual("ROT13 example.", Kata.Rot13("EBG13 rknzcyr."));

Not an M or an @ in sight. Also, M should transform to Z, not to @.

Not a kata issue.

First of all, why 'M' is '@'? Please look at the samples for C#. Next, In my IDE, character 'M' is translated into '@', but here it always stay 'Z'

You need to count also the ones that aren't repeated, 15/6 = 2.5. See the last example in the instructions.

The index is calculated based on how many vowels are repeated more than once in a row and dividing them by the total number of vowels a petty enemy of America would use.

"MEXICAAAAAAAANS GOOOO HOOOMEEEE", 2.50

Why do we have here 2.5? The number of extra vowels is 15, in 4 groups (according to description, we need to cound groups of vowels). When we divide 15 by 4, we get 3.75. How did you get 2.5?

yeah, is the error this one (testing") ?

Cool kata, but I think it should be 8 kyu

This isn't a kata suggestion, it's a question. If you order the numbers in increasing order, and you get the first 11, there is only a single 2.

Testing for [-7, 9, -8, -2, 2, 2, -4, -2, 1, -4, 7, 0, -8, -3, 9, -1, 10, -2, 6, 10, -5, 10, 0, 4, -2, 10, -2, -4, -7, 2, 4, -5] and 11
Expected: equal to [ -7, -8, -2, -4, -4, -8, -3, -5, -4, -7, -5 ]

Why is here on "expected" field I see "-7, -8, -2, -4, -4 ..."?
If I'm not mistaken, the answer should be "-7, -8, -2, -4, -2, -4 ..." as on the top we see that in a consecutive number -2 going before the second -4