Just a little heads up in case you didn't knew:
if you install packages like latte-dock from pacman (or build from source in this case) they will vanish with your next Steam Deck update because the Linux on the Steam Deck works quite different to a regular Linux installation
I wouldn't get so much hung up on latte-dock anyway since it is unmaintained since quite some time and doesn't even work on the latest KDE Plasma 6 (which SteamOS doesn't have yet but will come in the near future)
customizing the default Plasma Panel (right click on desktop > enter edit mode > add panel) is your best bet nowadays for a similar look
anyway if you are really dead set on latte dock you will need to "initialize" all public keys first from the Arch Linux and Steam developers until you actually can install anything on the package manager pacman
pacman-key --init
pacman-key --populate archlinux
pacman-key --populate holo
the last line is specific for SteamOS only