Not the first time I have encountered the fact that dependencies on the linux-image-amd64 package in the stretch-backports repository break. I see in the repository there is now only the linux-image-4.19.0-0.bpo.9-amd64-unsigned package dated 27-May-2020 18:28, and the required package linux-image-4.19.0-0.bpo.9-amd64 is not present.
In this way
root@test: ~ # apt install linux-image-amd64 -t stretch-backports
Reading package lists ... Done
Building dependency tree
Reading state information ... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
linux-image-amd64: Depends: linux-image-4.19.0-0.bpo.9-amd64 but it is not installable
E: Unable to correct problems, you have held broken packages.
How is it that packages breaking dependencies get into the repository and why are they not fixed for so long?