Installation | Signatures couldn't be verified

Hello there,
i'm trying to install omv 4 on debian 9 and i tried the codes in help, and the first one (which added the omv repository works fine, but after that when i run apt-get update i get errors on omv repositories saying the signature were invalid and it could not be verified and updating from such a repository can't be done securely and it's disabled, so i can't install omv!
what's wrong with that?

Quote from gderf

If you have the omv-extras.org testing repo enabled, disable it for now.

as i SAID, i'm trying to install OMV on debian, so no i don't have omv-extras.org.. it's the installation!