Loki 3.5.5, 3.5.4 and 3.4.6 do not have Debian or Red Hat packages available.
This is a problem for the APT repository (see https://apt.grafana.com/dists/stable/main/binary-amd64/Packages for a list of packages) and for the Github releases page (see Releases · grafana/loki · GitHub and check the assets). I couldn’t find any announcement that Debian or Red Hat packages would be discontinued, so I assume there has been a glitch of some sort.