root@imx8mn-ddr4-evk:~# apt update
Get:1 http://ftp.tw.debian.org/debian buster InRelease [122 kB]
Get:2 http://ftp.tw.debian.org/debian buster-updates InRelease [51.9 kB]                   
Err:1 http://ftp.tw.debian.org/debian buster InRelease                                       
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9 NO_PUBKEY DCC9EFBF77E11517
Err:2 http://ftp.tw.debian.org/debian buster-updates InRelease 
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9
Get:3 http://security.debian.org buster/updates InRelease [65.4 kB]
Err:3 http://security.debian.org buster/updates InRelease      
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 112695A0E562B32A NO_PUBKEY 54404762BBB6E853
Reading package lists... Done
N: Ignoring file 'debian-10.list.mod' in directory '/etc/apt/sources.list.d/' as it has an invalid filename extension
W: GPG error: http://ftp.tw.debian.org/debian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D17
E: The repository 'http://ftp.tw.debian.org/debian buster 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://ftp.tw.debian.org/debian buster-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACF9
E: The repository 'http://ftp.tw.debian.org/debian buster-updates 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://security.debian.org buster/updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 112695A0E563
E: The repository 'http://security.debian.org buster/updates 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.