For example, if your string contains both 1 and 0, and you replace 1 with I, that might create a situation where another part of the string (e.g., something containing 10) is now altered in an unexpected way when 0 is replaced with O.

It works perfectly for everyone. Not sure where you're getting an error.

If the digits can't be rearranged to form a bigger number, return -1

The only other number you can make with those digits is 12 and that's not bigger than 21.

If you meant the 21st test, those are random, print the input. But you should be able to see it even without printing it, because of how tests are made.

It's been answered several times already in Discourse - you need to read description (or just the title of the kata):

Primes that have only odd digits are pure odd digits primes

So 23 (which is a prime number, you are correct) is not a "pure odd digits prime" because one of its digits is not odd: 2 is an even number. You are looking for primes where ALL the digits are odd.

Please use spoiler flag next time.

'Max,John and Mark like this' should equal 
'Max, John and Mark like this'

Your string is missing a space after the comma.

don't forget to add the "a+b+c" option in your array =D

You're wrong, neither of those work.

Your code is wrong. It fails with a=1, b=1, c=1 for instance (you can see that in the sample tests). Why are you using loops at all? Drop the caps, please. And use spoiler flag next time too.

Read this: https://docs.codewars.com/training/troubleshooting/

you must use a spoiler flag when posting code, i added that flag just now