I tried following your instruction on debian stretch for rock64 but got the following errors:
jon@rock64:~$ sudo ./omvinstall.sh
Hit:1 http://security.debian.org stretch/updates InRelease
Hit:3 http://ppa.launchpad.net/ayufan/rock64-ppa/ubuntu xenial InRelease
Ign:5 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Hit:6 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease
Get:7 http://deb.ayufan.eu/orgs/ayufan-rock64/releases InRelease [1249 B]
Hit:8 http://cdn-fastly.deb.debian.org/debian stretch Release
Get:9 http://packages.openmediavault.org/public arrakis InRelease [12.7 kB]
Get:10 http://packages.openmediavault.org/public arrakis-proposed InRelease [6125 B]
Get:2 https://newcontinuum.dl.source…t/openmediavault/packages arrakis InRelease [12.7 kB]
Get:4 https://gigenet.dl.sourceforge…t/openmediavault/packages arrakis-proposed InRelease [6125 B]
Err:9 http://packages.openmediavault.org/public arrakis InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
Err:10 http://packages.openmediavault.org/public arrakis-proposed InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
Err:2 https://newcontinuum.dl.source…t/openmediavault/packages arrakis InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
Err:4 https://gigenet.dl.sourceforge…t/openmediavault/packages arrakis-proposed InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
Reading package lists... Done
W: GPG error: http://packages.openmediavault.org/public arrakis InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://packages.openmediavault.org/public arrakis InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://packages.openmediavault.org/public arrakis-proposed InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://packages.openmediavault.org/public arrakis-proposed InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://newcontinuum.dl.source…t/openmediavault/packages arrakis InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://downloads.sourceforge.net/project/openmediavault/packages arrakis InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://gigenet.dl.sourceforge…t/openmediavault/packages arrakis-proposed InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://downloads.sourceforge.net/project/openmediavault/packages arrakis-proposed InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package openmediavault-keyring
Hit:1 http://security.debian.org stretch/updates InRelease
Hit:4 http://ppa.launchpad.net/ayufan/rock64-ppa/ubuntu xenial InRelease
Ign:3 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Hit:6 http://cdn-fastly.deb.debian.org/debian stretch-updates InRelease
Get:10 http://deb.ayufan.eu/orgs/ayufan-rock64/releases InRelease [1249 B]
Get:7 http://packages.openmediavault.org/public arrakis InRelease [12.7 kB]
Hit:8 http://cdn-fastly.deb.debian.org/debian stretch Release
Get:9 http://packages.openmediavault.org/public arrakis-proposed InRelease [6125 B]
Ign:5 https://gigenet.dl.sourceforge…t/openmediavault/packages arrakis-proposed InRelease
Ign:2 https://phoenixnap.dl.sourcefo…t/openmediavault/packages arrakis InRelease
Err:7 http://packages.openmediavault.org/public arrakis InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
Get:12 https://phoenixnap.dl.sourcefo…t/openmediavault/packages arrakis Release [11.7 kB]
Get:11 https://versaweb.dl.sourceforg…t/openmediavault/packages arrakis-proposed Release [5155 B]
Err:9 http://packages.openmediavault.org/public arrakis-proposed InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
Get:14 https://versaweb.dl.sourceforg…t/openmediavault/packages arrakis-proposed Release.gpg [916 B]
Get:15 https://phoenixnap.dl.sourcefo…t/openmediavault/packages arrakis Release.gpg [898 B]
Ign:14 https://versaweb.dl.sourceforg…t/openmediavault/packages arrakis-proposed Release.gpg
Ign:15 https://phoenixnap.dl.sourcefo…t/openmediavault/packages arrakis Release.gpg
Reading package lists... Done
W: GPG error: http://packages.openmediavault.org/public arrakis InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://packages.openmediavault.org/public arrakis InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://packages.openmediavault.org/public arrakis-proposed InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://packages.openmediavault.org/public arrakis-proposed InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://versaweb.dl.sourceforg…t/openmediavault/packages arrakis-proposed Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://downloads.sourceforge.net/project/openmediavault/packages arrakis-proposed Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://phoenixnap.dl.sourcefo…t/openmediavault/packages arrakis Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7E7A6C592EF35D13 NO_PUBKEY 24863F0C716B980B
E: The repository 'http://downloads.sourceforge.net/project/openmediavault/packages arrakis Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package openmediavault is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'openmediavault' has no installation candidate
./omvinstall.sh: 14: ./omvinstall.sh: omv-initsystem: not found