It has been reported that users are unable to use the --encrypt option and or the --secure option via command line tools when using an embedded policy client.
When using the --encrypt command you will get the following error:
Operation start encrypt disk failed:
Error code -11984: Item not found
When using the --secure command you will get the following error:
Operation secure disk failed:
Error code -12220: Disk is already managed
This issue is fixed in the following release:
This version/Maintenance Pack is available for download via your account on Symantec File Connect.
Workaround
You can still encrypt the drive via the user interface (GUI) of the Symantec Encryption Desktop application.