could not find correct answer

Question

the 'tu' form of ganar in the subjunctive is gana not ganes.  It is only ganes in the negative.

Answer

Hola Elvira
This is the sentence you are referring to from the quiz:

Espero que ganes mucho dinero en el Euromillón.
I hope that you win a lot of money with Euromillions.

This form of the verb ganar "ganes" is the present subjunctive, not the imperative, and "gana" is the affirmative imperative form.

¡Gana mucho dinero! = Win lots of money! (imperative affirmative)
¡No ganes mucho dinero! = Don't win lots of money! (imperative negative that takes the form of the present subjunctive: gane, ganes, gane, ganemos, ganéis, ganen

I hope this clarified it.
Saludos
Inma