I can't get the piece after finishing the dungeon in normal and hard
There was no treasure there, only Ryo boxes

P.S. where can I use my Kijimuna ticket?

Treat the pieces like rare drops. Going solo and trying to get the pieces is a waste of your time. Go do the dungeon with a full party, although its still not a guarantee chance of getting the piece, you have a better chance.

Party size shouldn't affect the rates (Normal SS = 7.5%, Hard SS = 4.55%, Hell SS = 4.05%) but it may be quicker for some people. I don't quite trust CS's RNG, though, so it may be worth re-logging in every now and then if you aren't getting anything.

By the way, Kijimuna Tickets can be used in Onigashima by speaking to Ashiya Douman.