Restore from Backup Failed on GPGError

I attempted to restore from a backup I had on a USB hard drive using the PureOS Backup Tool, which is Déjà Dup Backup Tool 38.3. The backup is encrypted, but I was not prompted for the decryption key, and received the error below. How do you fix this problem?

GPGError: GPG Failed, see log below:
===== Begin GnuPG log =====
gpg: WARNING: "--no-use-agent" is an obsolete option - it has no effect
gpg: decrypt_message failed: Unknown system error
===== End GnuPG log =====

Try running duplicity on the command line with the verbose option to see why it fails. “Unknown system error” is not very specific.

@jjakob Thank you for the duplicity command. That got me the passphrase prompt and more useful information.