Maybe describing it in other wording:
https://forums.sketchup.com/t/lost-default-tray-on-2nd-monitor/148192/6?u=dezmo
Follow this guide exactly after opening SU
- Hold down the ALT key. Hit SPACEBAR.
- Release ALT key
- Hit M (Move).
- Hit one of ← ↑ → ↓ arrow keys
- Move your mouse around (without pressing any mouse key) until you see the missing window.
- Press left mouse button on the desired place
( Note I assume you have English windows otherwise you may need to replace “M” with a localized letter, e.g. “L” in French)