Alone in the Rainbow Forest

I didn't know what to do.

I chose the Rainbow Forest as starting point.

Because I saw lvl 1-3 below it

I opened a rainbow chest.

It contained <Gacha Materialization Crystal>

But it seems like I can't put it on my quick items usage interface. Basically, it has 7 slots and can be seen thru my mind. But the crystal can't be seen even in my inventory.