Welcome to THE OLD Switchroot L4T Ubuntu Linux Repo.

Please use the new repo at newrepo.switchroot.org!

Click here if you insist on seeing the instructions on setting this old, unused repo.
Import this key: https://repo.switchroot.org/pubkey

Add this line to your /etc/apt/sources.list (or somewhere in /etc/apt/sources.list.d):
deb https://repo.switchroot.org/ unstable/
(yes, you need to have that last slash)