Pull through cache does not find manifest

I have set-up registry as a pull through cache to ghcr.io as shown below.

  remoteurl: https://ghcr.io
  username: user
  password: ghp_pat

After I have done docker login successfully against this registry server, I am trying to pull an image from ghcr.io.

docker pull ghcr.io/a/b:latest
latest: Pulling from a/b
manifest unknown

What does this error message mean?

A number of answers suggested that, the tag may not be existing. The odd thing is, while I can see the container image in my repo, I can’t download even when logged out of my private registry (that is, connecting to ghcr.io directly without a proxy).

Verify that the tag latest exists, not all provide one which would result in such an error.