Если при установке ispmanager на старый debian/ubuntu получаем ошибку

W: GPG error: http://download.ispsystem.com/repo/debian 6-stable5-stretch InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 73CF0BC7810F8996
W: The repository ‘http://download.ispsystem.com/repo/debian 6-stable5-stretch InRelease’ is not signed.
N: Data from such a repository can’t be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://download.ispsystem.com/repo/debian base-stretch InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 73CF0BC7810F8996
W: The repository ‘http://download.ispsystem.com/repo/debian base-stretch InRelease’ is not signed.
N: Data from such a repository can’t be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://download.ispsystem.com/repo/debian stable5-stretch InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 73CF0BC7810F8996
W: The repository ‘http://download.ispsystem.com/repo/debian stable5-stretch InRelease’ is not signed.
N: Data from such a repository can’t be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Решить можно добавляем ключа

wget -q -O — http://download.ispsystem.com/repo/ubuntu/ispsystem.gpg.key | apt-key add —

wget -q -O — http://download.ispsystem.com/repo/ispsystem.gpg.key | apt-key add —

wget -q -O — http://download.ispsystem.com/repo/debian/ispsystem.gpg.key | apt-key add —