Welcome and thank you for your interest.
desktop repository contains the old Master Password desktop application, though the layout of the repo was slightly modified and no longer matches the gradle configuration, which wasn't brought up-to-date to match. This is because most likely, the Spectre redesign of the desktop app will no longer be based on the Java runtime.
This is the cause of your issues. I will see if I can restore the structure to a state where you can at least build the old Master Password desktop app, since it will still give you the same passwords that Spectre does.
The currently officially recommended method of using Spectre on a PC, would be to use the Spectre web application either from source (https://gitlab.com/spectre.app/web) or accessing it through: https://spectre.pw