connection time out
That looks like the Extensions Manager failing to reach the Slicer Extensions Catalog. The Slicer user guide lists two usual causes: the catalog server being temporarily overloaded (it recommends retrying after an hour or so), or the application not getting network access - Slicer uses the system proxy settings, so a proxy or firewall can block it. As an offline workaround you can download the extension package from the Extensions Catalog website in a normal browser and install it with the “Install from file…” button inside the Extensions Manager.