random tests added to C#, this solution does not pass anymore

Check this out: https://www.w3schools.com/python/python_for_loops.asp see what the example with the string "banana" prints.

for x in "banana":
  print(x) 

Now try this out, that's basically what you're trying to do with your code:

for x in "banana":
  print("banana"[x]) 

And see it'll fail with the same error that you get here.

in normal python it works though.

It doesn't work because it's wrong. And it won't work anywhere. Not a kata issue.

Because i is not a number. Please stop right now. Go read the Troubleshooting section of the docs I told you.

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

About your problem:

 for i in text:

i is a string there (a single char), you can't use text[i] to access a single letter of the string, neither i+1 makes sense.

I saw other people doing this

Look better at what they did, so you see how to correct your code.

That's a problem with your code and not a kata issue, again. Please, don't open issues every single time your code doesn't work if you're not sure if the problem is in the kata and not in your code. It's as the error says: string indices must be integers.

No, they shouldn't. What is i there? Your code being wrong is not a kata issue.

text[i] == 'w' or text[i] == 's' did not work, even though it should like in normal python.

I got an error that the system said is happening in the code, but I've seen others that can do it with my way. My code:
if text[i] == '_':
error:
string indices must be integers, not 'str'

I saw other people doing this, but mine didn't work, minor issue.