Me and most of my friends that have this game complete did it in task mode. I don't see how it wouldn't work in time attack mode but try it in task. Also they have to all be the same color not a combo over 50.
If you are having problems the easiest way to get 50 of the same color is start a task game of 3 color single. You have to fill up the right 3 collums to the top with nothing but 1 color. use the 2 left collums as a place to discard everything that is not 1 solid color. The 4th collum from the right is the same color as the rest of the right ones and you need need to leave the top 2 space open for the pieces that drop otherwise its game over. The third collum is just a buffer. You fill it up at the end to make 50. Also when filling the right side never push the x button as that will clear the screen.
Let me know if that helps or if I need to rephrase it better.