The command update-desktop-database is from package desktop-file-utils on debian which might not be installed, because kde doesn't need it, so kde users may not have this command available.

Related code : https://github.com/tauri-apps/plugins-workspace/blob/v2/plugins/deep-link/src/lib.rs#L333

Additional info:
You may want to use the command kbuildsycoca5 in kde environment.