Gfilui

root@BlackRishi98:~# apt-get update
Get:1 http://old.kali.org/kali sana InRelease [20.3 kB] 
Err:1 http://old.kali.org/kali sana InRelease 
 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
Get:2 http://ftp.yzu.edu.tw/Linux/kali kali-rolling InRelease [30.5 kB]
Err:2 http://ftp.yzu.edu.tw/Linux/kali kali-rolling InRelease
 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
Reading package lists... Done
W: GPG error: http://old.kali.org/kali sana InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
E: The repository 'http://old.kali.org/kali sana 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.yzu.edu.tw/Linux/kali kali-rolling InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
E: The repository 'http://http.kali.org/kali kali-rolling 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.

A few days back, I tried to install TheFatRat, a Kali Linux application. Thereafter I started getting these errors. I cannot even install any new packages.

Any help is appreciated.

It would appear that you added the Kali Linux Repository to your list of repositories to install that package, yet you did not add the repository's public key to your local keyring. Thus, Ubuntu cannot verify the authenticity of the repository.

The following commands should fix this problem:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys ED444FF07D8D0BF6
sudo apt update