Cross compiling for arm64

[lxxlxx]

Hello!
Please advise me with cross compiling lomiri for arm64.
I have already setup sbuild environment according to https://docs.ubports.com/en/latest/systemdev/testing-locally.html :

sudo apt install sbuild schroot devscripts debhelper dh-migrations dh-python ccache
sudo sbuild-adduser user
...
etc

But after executing sbuild --host=arm64 --build=amd64 -d "focal" i am getting this error:

E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal/main/binary-arm64/Packages  404  Not Found [IP: 91.189.91.82 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal-updates/universe/binary-arm64/Packages  404  Not Found [IP: 91.189.91.82 80] 
E: Some index files failed to download. They have been ignored, or old ones used instead.                                                    
E: apt-get update failed  

How can i fix that?