Why can't I chose a booster?

Hello Clare. There are some levels in the game which current allow you to use boosters.

That is just because they are introductory levels.

Here, level 4597 is an introductory level of crystal candy, which means one of the first levels which contains crystal candies(the candies which look like glass) .

Here's my current level and it doesn't allow me to use boosters as well. That is because this is the first level with introduction for Rainbow twists.