Interact with a mailbox to retrieve the Ninjago item from Cole, then give it back to Rocco Sirocco.
The mail from Cole will say:
"Hey Rocco! You left your Earth Spinjitzu kit back at the Monastery, so I’m sending it to you. Don’t leave this lying around again – we don’t want the Skeletons to get a hold of it!"
"There was a mix-up at the post office and my Ninjago package from Cole was sent to you by mistake! Could you get it from your mailbox for me?"
"Find Sensei Wu and you can learn Spinjitzu too. Watch this!"