The mentioned error appears when attempting to open LUKS2 (Linux Unified Key Setup) encrypted devices using PCManFM-Qt file manager in Lubuntu 18.04 LTS (Bionic Beaver). But it’s easy to solve, don’t worry.
a) Installing
Actually you need a package named libblockdev-crypto2.sudo apt install libblockdev-crypto2
b) Restarting
Then restart udisks2.service.sudo systemctl restart udisks2.service
The problem should be solved now. Try mounting the desired device again.