It’s not asking for password due the -background option.
You are adding (basically “initializing a second”) storage. Duplicacy is trying to create a new encrypted configuration (-e option).
I’ve never used the -background option with add, it doesn’t seem to make sense, since the password is not yet in the keychain. You would have to use an environment variable.
Try without -background option.
And I suggest evaluating, for your use case, if what you want is an add or a copy. The second creates a compatible storage. Look:
The -background option was meant to be an undocumented option – it was designed for the GUI only, since its purpose to suppress the prompt for entering password when no previous passwords are found in the environment or the keychain/keyring, which would lead to authorization errors (still better than hanging on waiting for user input).