我有 Lubuntu 16.04。我的问题类似于这个问题：
sudo apt-get update
Get:1 http://it.archive.ubuntu.com/ubuntu xenial InRelease [247 kB]
Hit:2 http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu xenial InRelease
Hit:3 http://it.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:4 http://it.archive.ubuntu.com/ubuntu xenial-backports InRelease
Hit:5 http://www.bchemnet.com/suldr debian InRelease
Hit:6 http://archive.canonical.com xenial InRelease
Get:7 http://security.ubuntu.com/ubuntu xenial-security InRelease [93,3 kB]
Fetched 340 kB in 6s (52,6 kB/s)
Reading package lists... Done
W: There is no public key available for the following key IDs:
FB510D557CC3E840
提供的答案均无效：
根据这个问题的答案#1：
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys FB510D557CC3E840
Executing: /tmp/tmp.oqit2axEyE/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys FB510D557CC3E840
gpg: requesting key 7CC3E840 from hkp server keyserver.ubuntu.com
gpgkeys: key FB510D557CC3E840 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: key not found
gpg: keyserver communications error: bad public key
gpg: keyserver receive failed: bad public key
或者来自同一问题的这个，请回答 #2：
sudo apt-get install debian-keyring debian-archive-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
debian-archive-keyring is already the newest version (2014.3).
debian-keyring is already the newest version (2016.01.20).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
或者来自同一问题的这个，请回答 #3：
sudo apt-get install debian-archive-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
debian-archive-keyring is already the newest version (2014.3).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
或者来自同一问题的这个，请回答 #4：
for n in `ls /var/lib/apt/lists/*gpg`; do echo "$n" ; gpg --list-packets "$n" | grep FB510D557CC3E840; done
ls: cannot access '/var/lib/apt/lists/*gpg': No such file or directory
或者来自同一问题的这个，请回答 #5：
sudo apt-get install debian-keyring debian-archive-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
debian-archive-keyring is already the newest version (2014.3).
debian-keyring is already the newest version (2016.01.20).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
再试一次，回答这个问题的#1：
gpg --keyserver keyserver.ubuntu.com:80 --recv-keys 7CC3E840
gpg: requesting key 7CC3E840 from keyserver.ubuntu.com:80
gpgkeys: no keyserver host provided
gpg: keyserver internal error
gpg: keyserver receive failed: keyserver error
回答同一个问题的第 2 个：
sudo gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 7CC3E840
gpg: WARNING: unsafe ownership on configuration file `/home/username/.gnupg/gpg.conf'
gpg: external program calls are disabled due to unsafe options file permissions
gpg: keyserver communications error: general error
gpg: keyserver receive failed: general error
确保没有权限问题：
ls -l ~/.gnupg/gpg.conf
-rw------- 1 username username 9398 mag 16 15:34 /home/username/.gnupg/gpg.conf
从上一个答案：
apt-key adv --keyserver hkp://keyserver.ubuntu.com --recv-keys 7CC3E840
Executing: /tmp/tmp.8iRzKu4RVN/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com --recv-keys 7CC3E840
gpg: requesting key 7CC3E840 from hkp server keyserver.ubuntu.com
gpgkeys: key 7CC3E840 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: key not found
gpg: keyserver communications error: bad public key
gpg: keyserver receive failed: bad public key
缺少的密钥是 Samsung Unified Linux Driver Repository 的密钥。按照昨天添加到网站的说明应该可以解决问题：
wget http://www.bchemnet.com/suldr/pool/debian/extra/su/suldr-keyring_1_all.deb
sudo dpkg -i suldr-keyring_1_all.deb
（当然，假设您信任存储库！）。