Installation on ubuntu for command line usage

I would like my Ubuntu’s NAS (OMV) to run cryptomator and perform an encrypted backup of my data online. As I have only command line available I went for the instructions found at this page:

however the repository installation appears to fail.

root@NAS:~# sudo add-apt-repository ppa:sebastian-stenzel/cryptomator
 Cryptomator provides free client-side AES encryption for your cloud files. Create encrypted vaults, which get mounted as virtual volumes. Whatever you save on one of these volumes will end up encrypted inside your vault.
 More info:
Press [ENTER] to continue or ctrl-c to cancel adding it

gpg: keybox '/tmp/tmppi3vsm9t/pubring.gpg' created
gpg: key 892C15CD130FEB9A: 1 signature not checked due to a missing key
gpg: /tmp/tmppi3vsm9t/trustdb.gpg: trustdb created
gpg: key 892C15CD130FEB9A: public key "Launchpad PPA for Sebastian Stenzel" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
gpg: no valid OpenPGP data found.

Am I doing the right thing here? Is there perhaps a better way to achieve what I’d like to do on a system with no graphical interface?