[2019] Trouble with Supply Challenge

I decided to give the Supply Challenge scenario a shot, but it doesn't appear to work.

For the first level, you need to obtain 15 Stone. I've done that and put it in the provided red chests, but nothing seems to happen (and I'm met with a Game Over when the timer hits 0). I've tried stone bricks instead, but still nothing. It also doesn't seem to matter which chest I put it in.

I don't know whether I'm doing something wrong, or if there's a bug present. Does anyone here happen to know what's going on?