Xenial: Package 'firmware-amd-graphics' has no installation candidate

This is about a system migration, where a Foxconn motherboard died and was replaced with an Asus, same CPU and RAM, but a lot of differences. Some necessary reconfiguration is obviously missing, as ip a finds only the loopback device, /dev/dri/ is missing, vtty framebuffers are using 1024x768, and X is using FBDEV instead of modesetting or radeon.

dmesg
Xorg.0.log

/etc/sources.list:Code:# deb http://us.archive.ubuntu.com/ubuntu/ xenial main restricted

# deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
# deb http://security.ubuntu.com/ubuntu xenial-security main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ xenial main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ xenial universe
deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ xenial multiverse
deb http://us.archive.ubuntu.com/ubuntu/ xenial-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu xenial-security main restricted
deb http://security.ubuntu.com/ubuntu xenial-security universe
deb http://security.ubuntu.com/ubuntu xenial-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu xenial partnerCode:# apt install firmware-amd-graphics
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package firmware-amd-graphics is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'firmware-amd-graphics' has no installation candidate
#This looks like the crux of the problem:Code:Module Size Used by
ata_generic 16384 0
pata_acpi 16384 0
serio_raw 16384 0I don't understand how this could be given the nearly 40MB size of each of the initrds, and installed headers and modules packages for each installed linux-image.

Debian Stretch on same PC is working just fine, while Bionic is similarly deficient WRT GPU and firmware-amd-graphics, but has functional networking. Can someone please tell me what I've done wrong, or not done, that apt cannot find needed firmware, and most needed modules are not getting loaded?