Your hand may indeed be a valid hand. However, the way the exercise works is that the computer picks a random hand, removes a certain number of tiles and looks for you to guess that particular hand.
It's possible that a given puzzle matches different hands or variations of that hand (especially when the "hard" difficulty level is chosen, as there will be more blank tiles) but the goal of the game is to match the ONE hand that the computer chose, not just any variation that fits (this is why players are given several attempts).
Just like in Wordle, where you have to guess THE word chosen, not just any word that fits.