10 or earlier installed, you must first upgrade to macOS Mojave 10.
It uses a strong encryption method, which makes it impossible to calculate the key in any way. If you are convinced you don't want it, the reason it is hidden is it partition 'id' type - and not being able to set the drive letter is another symptom, not a cause. If you have not installed a web service or configured a fire wall before, you should seek help from a qualified network administrator prior to doing so. If you already have a folder within them, drag that to the Desktop. The provided credentials could not be validated. Also – these steps do not include the hardening of your BIOS settings – passwords and the likes.
