When are regular season single tickets available?

Hi, I am from another country and haven´t been to any MLB game in my life. I will be visiting Houston during the regular season and I would like to buy single game tickets, but when I want to select a date, it does not let me select any date of the regular season. Does this mean all the season is sold out or are they not for sale yet?

Button of \"Buy Tickets\" doesn´t do anything