Welcome to the KNIME Forum.
It looks like you’re installing a third-party community extension, which is not hosted by KNIME. You will find installation instructions from the vendor directly, see here.
Should you be looking for the KNIME AI extensions, you will find them here:
I am rather clueless why the update site disappeared from your KNIME configuration. To re-add it using the classic ui, go to File → Preferences → Install/Update → Available Software Sites, click on Add… and paste the update site URL.