Without any information, we do not know either. Sorry.

I've tested your solution and it passed both sample and real tests without compilation errors!

I believe you're not allocating enough memory for the string (for example, check case 3 more closely).

Assert::That(highestScoringWord("what time are we climbing up the volcano"), Equals("volcano"));

The expected answer for that test is "volcano", not "climbing". That's a problem in your code, not a kata issue.

Did you read this :" where 0 <= m <= n or m >= n >= 0" ? No errors in the tests. Read at the top of the page how many people passed the C++ kata.