Открытый ключ не найден на keyserver.ubuntu.com при попытке выполнить обновление sudo apt-get

Я новичок в Ubuntu, я получил следующее сообщение об ошибке при попытке обновить apt-get:

(base) william@william-hp-spectre:~$ sudo apt-get update
[sudo] password for william: 
Hit:1 http://us.archive.ubuntu.com/ubuntu bionic InRelease
Get:2 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]   
Hit:3 http://ppa.launchpad.net/atareao/atareao/ubuntu bionic InRelease         
Get:4 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]    
Hit:5 https://packages.microsoft.com/repos/vscode stable InRelease             
Ign:6 http://hyperrate.com/gcin-ubuntu1804 eliu InRelease                      
Get:7 http://us.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB] 
Hit:8 http://ppa.launchpad.net/nomacs/stable/ubuntu bionic InRelease           
Hit:10 http://hyperrate.com/gcin-ubuntu1804 eliu Release                       
Get:9 http://debian.luna.com.tw/ubuntu ./ InRelease [4,022 B]                  
Err:9 http://debian.luna.com.tw/ubuntu ./ InRelease            
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7ACEB47B5A4CEB4C
Reading package lists... Done
W: GPG error: http://debian.luna.com.tw/ubuntu ./ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7ACEB47B5A4CEB4C
E: The repository 'http://debian.luna.com.tw/ubuntu ./ 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.

Затем я попытался найти ключ 7ACEB47B5A4CEB4C на keyserver.ubuntu.com, он показал:

(base) william@william-hp-spectre:~$ gpg --keyserver hkp://keyserver.ubuntu.com --search-keys 7ACEB47B5A4CEB4C
gpg: error searching keyserver: No data
gpg: keyserver search failed: No data

Я перечислил все доверенные ключи на моем Ubuntu 18.04 LTS:

/etc/apt/trusted.gpg
--------------------
pub   rsa2048 2012-05-03 [SC]
      3F1A 3C3A 00BA 96E0 5789  B950 5C57 7D59 835A B0E3
uid           [ unknown] Edward Liu (my gpg) <eliu@hyperrate.com>
sub   rsa2048 2012-05-03 [E]

pub   rsa2048 2015-10-28 [SC]
      BC52 8686 B50D 79E3 39D3  721C EB3E 94AD BE12 29CF
uid           [ unknown] Microsoft (Release signing) <gpgsecurity@microsoft.com>

pub   rsa4096 2017-01-17 [SC] [expired: 2019-01-17]
      6BA2 B0F3 B8B0 6169 7E98  ADFA 6D37 89ED C340 1E12
uid           [ expired] Vivaldi Package Composer KEY03 <packager@vivaldi.com>

pub   rsa4096 2018-01-05 [SC] [expires: 2020-01-25]
      68AE AE71 F9FA 1587 03C1  CBBC 8D04 CE49 EFB2 0B23
uid           [ unknown] Vivaldi Package Composer KEY04 <packager@vivaldi.com>
sub   rsa4096 2018-01-05 [E] [expires: 2020-01-25]

/etc/apt/trusted.gpg.d/atareao_ubuntu_atareao.gpg
-------------------------------------------------
pub   rsa1024 2010-12-14 [SC]
      A3D8 A366 869F E2DC 5FFD  79C3 6A96 53F9 36FD 5529
uid           [ unknown] Launchpad PPA for atareao

/etc/apt/trusted.gpg.d/microsoft.gpg
------------------------------------
pub   rsa2048 2015-10-28 [SC]
      BC52 8686 B50D 79E3 39D3  721C EB3E 94AD BE12 29CF
uid           [ unknown] Microsoft (Release signing) <gpgsecurity@microsoft.com>

/etc/apt/trusted.gpg.d/nomacs_ubuntu_stable.gpg
-----------------------------------------------
pub   rsa1024 2012-01-20 [SC]
      5467 8F40 F5AC 726E 3CE0  3538 D99B 1D26 2993 5E8D
uid           [ unknown] Launchpad PPA for nomacs developers

/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-archive.gpg
------------------------------------------------------
pub   rsa4096 2012-05-11 [SC]
      790B C727 7767 219C 42C8  6F93 3B4F E6AC C0B2 1F32
uid           [ unknown] Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>

/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-cdimage.gpg
------------------------------------------------------
pub   rsa4096 2012-05-11 [SC]
      8439 38DF 228D 22F7 B374  2BC0 D94A A3F0 EFE2 1092
uid           [ unknown] Ubuntu CD Image Automatic Signing Key (2012) <cdimage@ubuntu.com>

/etc/apt/trusted.gpg.d/ubuntu-keyring-2018-archive.gpg
------------------------------------------------------
pub   rsa4096 2018-09-17 [SC]
      F6EC B376 2474 EDA9 D21B  7022 8719 20D1 991B C93C
uid           [ unknown] Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>

Как я могу найти или добавить ключ, который не существует на сервере ключей?