There isn't really any mistake in kata that I can find. Yes, your code should work for big numbers, because it's 5kyu. You are looping up to n / 2, but can you do better?

For your 2nd point, yes, most tests expect nil (but not always), because generated numbers are simply random. For this reason I will keep the issue open, if the author chooses to address it, but overall the kata "works".

There is a mistake in random tests in ruby. In one point it's expected the code to pass with really big numbers, over 2 billions, at second poing it's expected to return nil for number over 1 billion becouse its to big number.

ah yes, thanks!

Read this: https://docs.codewars.com/training/troubleshooting#print-input