hey david,
that's happening because you're on 3.8 and the version you're trying to update to is 3.9.
the auto-updater only works in the same branch. so when you have 3.9.x you can always use the auto-updater to the latest 3.9.x.x.x version but not to 4.0 then.
to upgrade to a new major version you need to upload the package manually via the extension installer, that will work fine.
hope that helps