You have to do content for spaces that are not darkened. After completion, the space will light up and have a flashing arrow. Then you click it to get a sticker and darken the space.

If you want to get credit for the same space multiple times, you have to Second Chance > Retry to undarken the space first. This will simultaneously darken a random space so that the number of stickers is always equal to the number of darkened spaces.