Because they aren't in the input, read the error message in the Attempt tests or see the sample tests:

test.assert_equals(delete_nth([20,37,20,21], 1), [20,37,21]) # your code passes this one
test.assert_equals(delete_nth([1,1,3,3,7,2,2,2,2], 3), [1, 1, 3, 3, 7, 2, 2, 2]) # but not this one

You have a bug in your code. Why have you added an argument to the function?