Two problems that look different and have one cause each.
"Not installed", but you have it
The app looks for each CLI in the places installers actually use — Homebrew's bin directories, the user's local bin, the vendor's own install path — and asks your login shell as a fallback. A tool installed only into an interactive shell's startup file can be missed, because a login shell does not read that file.
Open Settings → Who's In The Room and press Test on that seat. That re-runs detection and reports where it looked and what it found. If the CLI is somewhere unusual, put its directory on your PATH in the login shell's startup file, or install it with the vendor's installer so it lands where the app looks.
The ltb command hangs, or the editor extension shows nothing
Quit the LetThemBuild app first. The command line runs the app without its window, and the two share one settings and session store, so running both at once leaves them waiting on each other. Run ltb with the app closed, or use the app with the terminal closed.
The extension cannot find ltb
Install the command from Settings → Workspace & Account → Command line → Install the command…. On a Mac it goes to /opt/homebrew/bin or /usr/local/bin; on Linux the deb and rpm put it in /usr/bin; on Windows it is ltb.cmd in the WindowsApps folder. If it lives elsewhere, set letthembuild.cliPath in the editor's settings.
Still stuck
Send us a message from this help centre with the seat's Test output, or the version line from About and what you tried. See "How to reach support".