You’ll need to switch to Desktop Mode to access the GUI and Terminal:

• If you haven’t already, use passwd to create a password for the deck user.

• Disable read-only mode: sudo btrfs property set -ts / ro false

• Initialize the pacman keyring: sudo pacman-key --init

• Populate the pacman keyring with the default Arch Linux keys: sudo pacman-key --populate archlinux

• Try installing a package: sudo pacman -S vi

NOTE:
Any packages you install will be overwritten by the next Steam Deck update.  You can maintain persistence by saving your content under /run/media/mmcblk0p1/ (your microSD card).

For example, I keep my Discord Rich Presence client, RetroArch/EmuStation, and all my content under /run/media/mmcblk0p1/Emulation and /run/media/mmcblk0p1/home/deck so that neither are overwritten or lost during SteamOS updates.

config.json

{
    "STEAM_API_KEY": "YOUR_STEAM_API_KEY", # You can register at https://steamcommunity.com/dev/apikey
    "USER_ID": "YOUR_USER_ID", # You can get the ID from the URL ID returned by https://steamdb.info/calculator/</code>

    "DISCORD_APPLICATION_ID": "1014501243164577802",

    "COVER_ART": {
        "ENABLED": true,
        "STEAM_GRID_API_KEY": "8a65c99870dc888583e06c3e0f377560"
    },

    "CUSTOM_GAME_OVERWRITE": {
        "ENABLED": false,
        "NAME": "a game on Steam Deck"
    },

    "CUSTOM_STATUS_STATE": {
        "ENABLED": false,
        "STATUS": "using Discord on Steam Deck"
    },

    "CUSTOM_ICON": {
        "ENABLED": false,
        "URL": "https://tayledras.com/wp-content/uploads/2022/08/steamdeck-logo.png",
        "TEXT": "Discord on Steam Deck"
    }
}