. Considering your tank can't even be assembled, I would recommend sending an email to Hasbro asking if they have a replacement ring piece or even the whole chassis. Worst case scenario is they offer you a replacement vehicle if you send yours in.
Ahh, I didn't realize that's what the backpacks were for, and I didn't get what to do with the mines at all..
I actually did send a note to Hasbro, but their response team only works weekdays, which just means I had a day and a half to grump about it. I'm thinking they'll somehow try to arrange a replacement piece, or offer to replace the whole toy if I send it in. From what I've seen they tend to be responsive to this type of mistake.