In order to sync one or more “distributions” from a Debian repository, we need the path to the “Release file”. The basepath is constructed as follows:
<upstream_url>/dists/<distribution>/
The <distribution> part is one of the elements that the Katello user entered into the Releases: field in the Katello UI. In your case it is stretch or stable. Once the base path has been constructed the sync will look for three files in this location Release, InRelease, and Release.gpg at least one of Release or InRelease must exist for the sync to continue.
So in your case we want the following:
http://ftp.debian.org/debian/dists/stretch/Release
Which exists! The sync should be possible.