This is something I've never seen before, so I'm not sure what Word thinks is wrong. Its suggestions look like the randomish stuff I'd expect from a word not in the dictionary, but that would be a red underline. The standard complaint about contractions would be green, so that's not it either.
The sentence in question is:
My calendar is more or less free all day, so feel free to call whenever’s good for you.
The underline is on whenever's