GPG can't check signature

First of all, you should import the key to local keyring as @enzotib instructed:

gpg --keyserver keyserver.ubuntu.com --recv-keys 7ADF9466

Then export the key to your local trustedkeys to make it trusted:

gpg --no-default-keyring -a --export 7ADF9466 | gpg --no-default-keyring --keyring ~/.gnupg/trustedkeys.gpg --import -

I believe the conventional solution is to install the GnuPG keys of Debian Developers package:

sudo apt-get install debian-keyring

You should import the key to local keyring with the following command:

gpg --keyserver keyserver.ubuntu.com --recv-keys 7ADF9466

Then, try again the command.

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