Not a kata issue.

Your code isn't failing on "", it's failing on a different input. Look more closely... you fail to account for this input in your code.

Reread the description. You get "wrong" input, yes, but you have to correct that input to get the result :)

It will shuffled up to digit. Not only numbers.

Yeah, same.

The answer is 4:16, but what you return is 4:12.
Your solution works fine now, so this is not a problem for you anymore, I think.

Please use a spoiler flag when you post code. Also use markdown tags to format your code or it's not usable. Refer to the documentation about this and more: https://docs.codewars.com/training/troubleshooting/#post-discourse

Please try to be clever: your code fails on some tests, you can easily see that your code is incorrect with those inputs (example isCoprime(17, 34): expected true to equal false). Draw the necessary conclusions.