Why when you have won a star box and you try to purchase one of the rewards offered which can only..

If I understand correctly, you are referring to opening a pink (5-star) box and receiving a purple Marvelous card with a reward decoration. When you claim the reward, it goes to the store. Tap on the either the collection set icon or open the store's 4th tab and look for the decoration. The decoration will now say "place" instead of taking you to the star box. Tap "place" to place it on your island.

If this is not happening, then you should contact customer support through the "help" button in the game settings (green gear icon).

I played the entire Halloween episode. Never did get a purple marvelous reward. This is not the first time this has happened.