This package contains in deb http://ftp.de.debian.org/debian buster main repo which is not available in ubuntu.
First you need to do add this to /etc/apt/sources.list like below
#for kubectlx
deb [trusted=yes] http://ftp.de.debian.org/debian buster main
Then run
sudo apt-get update
Then you can install using this cmd.
sudo apt install kubectx