If the "piece" refers to a mechanical puzzle involving statues, you must rotate them to line them up with the boxes located near each statue to open nearby doors.
If you are stuck on a specific "piece" or puzzle in the game, here are the most common solutions based on player guides: Common Progression Blocks LustHunter-0.6.4-pc.zip
Community-driven guides and help can often be found on the game's Itch.io discussion boards where players share specific solutions for puzzle keys and quest items. If the "piece" refers to a mechanical puzzle
To progress past certain points in this version, you must visit the new "King Place" and interact with the King to initiate specific story flags. LustHunter-0.6.4-pc.zip