This was a known limitation in the Glass layout that shipped with Cursor 2.7.0 — the secondary sidebar was temporarily disabled by the new UI.
Good news: a fix has been shipped. Other users in a related thread confirmed that extensions can now be docked to the right sidebar again after a recent update.
Since you’re on the nightly track, you should get this fix with your next update. If you’re still seeing the issue after updating, try running View: Reset View Locations from the Command Palette, then Developer: Reload Window.