Tap/click on the ? to get the rules. I assume you're trying to play something that's not in the dictionary, so you're getting the message I'm showing you here. If that's not what you meant, or you have other questions, feel free to ask :)
there's a book icon on the top left, that's the dictionary, and what it's saying is only words u're able to look up on the dictionary are words u're able to enter into shiritori cat