The Platinum ticket camp equipment (Winter Builder Winner 3) is 2-wide x 4-high . You have to have 2 slots open in one of the 4-high rows in the camp underground in order to place the item in the underground. (You can't buy the item unless you can place it.) You need to open the slots that the game is showing you.
It isn't necessary to have the 3-high rows cleared first. You can just scroll down past the 3-high rows until you get to the first 4-high row and open the 4-high slots if you have that materials or gems.