To open your chest, you need to place it on your board first. Tap on your chest to place it on your board.

You will see an '!' on your chests when you place them on your game board. It means that you need to click the 'Open' button to start the cooldown period. The waiting time varies by chest level.

You can wait for the cooldown period, or you can skip it by using your gold. At the end of the cooldown period, you can tap on your chest and spawn items.