$ rmadison linux-image-generic-hwe-16.04 linux-image-generic-hwe-16.04 | 4.13.0.37.56 | xenial-security | amd64, arm64, armhf, i386, ppc64el, s390x linux-image-generic-hwe-16.04 | 4.13.0.37.56 | xenial-updates | amd64, arm64, armhf, i386, ppc64el, s390x linux-image-generic-hwe-16.04 | 4.13.0.38.57 | xenial-proposed | amd64, arm64, armhf, i386, ppc64el, s390x
So it's there. I guess you need to change your mirror.
$ rmadison linux-image- generic- hwe-16. 04 image-generic- hwe-16. 04 | 4.13.0.37.56 | xenial-security | amd64, arm64, armhf, i386, ppc64el, s390x image-generic- hwe-16. 04 | 4.13.0.37.56 | xenial-updates | amd64, arm64, armhf, i386, ppc64el, s390x image-generic- hwe-16. 04 | 4.13.0.38.57 | xenial-proposed | amd64, arm64, armhf, i386, ppc64el, s390x
linux-
linux-
linux-
So it's there. I guess you need to change your mirror.