If you have a (local connected) LUKS encrypted USB partition/drive, then Clonezilla is unable to use/open it as a backup target.
It would be nice if Clonezilla could detect that the target drive is encrypted using LUKS and would ask the user for the unlock passphrase.
As soon as the target drive/partition is unlocked, the normal backup process can resume.
This feature request is different from feature request #39, because that issue suggests using GPG to encrypt files, and this feature request would implement writing to a LUKS encrypted partition.
The encrypted target can be unlocked using open source tools (cryptsetup).