Please verify that when you uploaded the signing keys into Artifactory, you pressed the "Save" button located next to the pass-phrase. This can be verified by looking for the following snippet in your Config Descriptor in the Admin tab on the Artifactory UI ? Advanced ? Config Descriptor:
If you are not able to locate those lines, then it means the pass-phrase was not saved to the configuration.
This needs to be verified because if the pass-phrase was not saved, Artifactory does not generate the "Release.gpg" file upon calculating the Debian indexes, which is required by the Debian client in order to authenticate the packages downloaded from Artifactory.
After verifying that it has been saved, re-trigger the calculation of the Debian indexes by pressing on the Debian repository on the list of repositories inside the Admin tab ? Repositories, and by going to the "Packages" tab on this dialog and pressing "Recalculate Index" under the "Debian" section.
After doing that, make sure to verify the presence of the "Release.gpg" file on the Debian repository in the Artifactory Tree Browser.
After making sure everything is in place, run the apt-get update operation on your machine and try downloading the package again.