E: The repository 'http://deb.globaleaks.org focal/ Release' is not signed

Hi,
trying to install glk on Ubuntu 20.04 I obtained this error:
root@glk:~# ./install-globaleaks.sh
Running the GlobaLeaks installation…

E: The repository ‘http://deb.globaleaks.org focal/ Release’ is not signed.

How can I resolve?

ML