ホームサーバーでこれを実行すると、次のようなapt update結果が得られます。
Hit:1 http://ftp.debian.org/debian unstable InRelease
Get:2 http://ftp.ports.debian.org/debian-ports unstable InRelease [55.3 kB]
Get:3 http://ftp.ports.debian.org/debian-ports unreleased InRelease [56.6 kB]
Ign:4 https://download.owncloud.org/download/repositories/stable/Debian_9.0 InRelease
Hit:5 https://download.owncloud.org/download/repositories/stable/Debian_9.0 Release
Err:2 http://ftp.ports.debian.org/debian-ports unstable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A88D659DCB811BB
Err:3 http://ftp.ports.debian.org/debian-ports unreleased InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A88D659DCB811BB
Fetched 112 kB in 3s (33.5 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
765 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ftp.ports.debian.org/debian-ports unstable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A88D659DCB811BB
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ftp.ports.debian.org/debian-ports unreleased InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A88D659DCB811BB
W: Failed to fetch http://ftp.ports.debian.org/debian-ports/dists/unstable/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A88D659DCB811BB
W: Failed to fetch http://ftp.ports.debian.org/debian-ports/dists/unreleased/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A88D659DCB811BB
W: Some index files failed to download. They have been ignored, or old ones used instead.
他の質問によると、次のように修正する必要があります。
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 5A88D659DCB811BB
ただし、次のエラーが発生します。
Executing: /tmp/apt-key-gpghome.fTPsBznh8Q/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 5A88D659DCB811BB
gpg: keyserver receive failed: No data
この問題をどのように解決できますか？
編集する
コメントで提案されているようにDebianを試してください。
apt-key adv --keyserver keyring.debian.org --recv-keys 5A88D659DCB811BB
Executing: /tmp/apt-key-gpghome.HQ0bpUZIjb/gpg.1.sh --keyserver keyring.debian.org --recv-keys 5A88D659DCB811BB
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0