Reference
Troubleshooting
What can go wrong, and the fix.
The button is not in the toolbar
Enable the plugin under Edit > Plugins and restart the editor. If it is already enabled, look to the right of the play controls.
The entries are greyed out
You are in Play In Editor. Stop the session.
"could not be found on disk"
The level was moved, renamed or deleted after you added it. Pick it again in the settings, or remove the row.
My list is different from a teammate's
Config/DefaultQuickLevelManagerSetting.ini was never committed, or one of you has unpushed changes to it.
Two entries have the same name
Two levels in your project share an asset name. Hover each one to see its full package path.