Troubleshooting

What you see	What to do
Not in the Extensions menu	Quit REAPER completely and start it again. Check there is exactly one reaper_analyser file in ~/.config/REAPER/UserPlugins.
Docked at the bottom	Right-click the Analyse 1 tab, dock it at the top of the main window, then drag the docker edge to size it. REAPER remembers both.
The meters do not move	Check REAPER is playing through your audio device, that Analysis enabled is ticked in the menu, and that no module is frozen.
Spectrum shaded at the top	Check Rate in Performance. Bluetooth headphones often drop to 16 kHz when a microphone is in use; use wired headphones or speakers.
Quieter, mono or bass-heavy	Dim, Mono or LF solo is on; the orange bar says which. Click it again, or hide and show the panel.

Still stuck? See Support, and send the version and build date from the Brand module.