Sandbox is available only when its module is plugged in on the Main Menu. The module can be found by the Reclamation Bin in the Museum. After reclaiming it and completing the scene, it will appear in a Bonebox. it can then be plugged in to a socket near the left wall to unlock sandbox as a mode of play.

What modes does Boneworks have?

The game has three game modes: Story mode, Arena mode, and Sandbox mode. The Arena mode unlocks after you finish the story and you can unlock the Sandbox mode earlier in the game in a specific way, but I won't spoil that for you.

Is Boneworks sandbox on PSVR?

Boneworks is a physics sandbox that allows players to tackle situations in almost any way they see fit. ... It's possible that Project 4, an in-development game featuring “Boneworks​' mechanics and core systems,” will come to PSVR on PS4.