The second half of my previous comment says how to know what the test case is.

On the sample tests it worked perfectly, but I don't even know the case on which it failed on the attempt.

When you find a test your code fails on you should be debugging that particular test case, not other ones.
If you weren't already aware, you're free to print out anything you like to see, for example the input.

i don't understand:

expected True but was False

I have tested it on my own tests many times and I can't think of what could be wrong with my solution

Sample tests don't follow the pattern given in the description.

No fixed tests.

The tests shouldn't log anything.

Because your solution is wrong.

Randomness isn't checked: https://www.codewars.com/kata/reviews/5fd23ad1b68b7c00015ca6d7/groups/5fd25b5071a9d80001ffe149

What's the point of passing Random to this function?

The tests are incorrect. For example sample tests:

const string example = "Hello, my {friend|enemy}";
var result = StringRandomizer.RandomizeString(example, Rand);
Assert.IsTrue(example == "Hello, my friend" || example == "Hello, my enemy");

example is const and never changes, so how could it equal something else?

I hope this isn't the kind of check that's used in final tests... Are there any tests for randomness?

Looks like resolved ;-)

Now it is also solved in cw-DB. :-)

Great thank you! :D