clara repo is unable to read and import the private key because in the version of GPG on Scibian9 (>=2.1) they removed the secret keyring and let only gpg-agent has the control of the private part of key as explained here : https://www.gnupg.org/faq/whats-new-in-2.1.html#nosecring.
These modifications has a major impact on clara repo key and clara repo init, without a public/private key policy the initialization of repos or checking the keys give errors