porque me cobran 99,99 € cuando lo que queria era la de 12 € mensuales
If you wanted monthly instead of yearly, you should contact support. However, the monthly subscription isn't 12, it's more. When paying a year in advance you get a discount over the amount you would pay by doing it month to month.
https://support.chess.com/article/346-contact-us
porque me cobran 99,99 € cuando lo que queria era la de 12 € mensuales