returns in your code are not structured correctly, and this causes a couple of problems:

• post-iteration statement c++ in your inner loop is never reached, because your inner loop always hits break or return on the first iteration. Your inner loop never has a chance to execute more than once.
• more serious problem though is that return can be completely missed if sentence would happen to be empty. Your inner loop will never enter, and return won't be encountered by the control flow. Compiler does not like this, you have to have some return statement which will be executed even if your for loops won't be entered.

After seeing your code, it's not a kata issue, it is a problem with your code.

Read this: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution

Given it's one of your firsts katas and

C# Completions 2646

It's probably you who is doing something wrong. Don't open an issue in cases like this, use question label and provide your code using markdown formatting and marking your post as having spoiler content, otherwise there is no way to help you.