All arm64 packages ONLY exist on the ports.ubuntu.com repositories. Your errors are because you are using the standard archives - which only carry i386 and amd64 architechtures - and will be breaking.
Based on your comment indicating your system architecture is x86_64, you need amd64 packages, and NOT arm64 packages. Why your system is searching for ARM64 packages is beyond me, so we're going to fix that. So let's fix that problem first, because you shouldn't need arm64 packages on a standard install.
Remove arm64 from foreign architectures that dpkg/apt looks for.
sudo dpkg --remove-architecture arm64
Reset your sources list back to previous.
Use something like this:
deb http://gb.archive.ubuntu.com/ubuntu focal main restricted universe multiverse
#deb-src http://gb.archive.ubuntu.com/ubuntu focal main restricted universe multiverse
deb http://gb.archive.ubuntu.com/ubuntu focal-updates main restricted universe multiverse
#deb-src http://gb.archive.ubuntu.com/ubuntu focal-updates main restricted universe multiverse
deb http://gb.archive.ubuntu.com/ubuntu focal-backports main restricted universe multiverse
#deb-src http://gb.archive.ubuntu.com/ubuntu focal-backports main restricted universe multiverse
deb http://gb.archive.ubuntu.com/ubuntu focal-security main restricted universe multiverse
#deb-src http://gb.archive.ubuntu.com/ubuntu focal-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu focal partner
#deb-src http://archive.canonical.com/ubuntu focal partner
(Note I commented out the deb-src lines, unless you're doing things like apt source calls or such you probably don't need them).
Run sudo apt update again. You should see no more errors.