After updating from 3.0-RC2 to 3.01-RC8, "dcs not available", same error.
(uploaded Duet3Firmware_MB6HC.bin on Webmenu / System).

Repaired with:
wget -q https://pkg.duet3d.com/duet3d.gpg
wget -q https://pkg.duet3d.com/duet3d-unstable.list
sudo mv duet3d.gpg /etc/apt/trusted.gpg.d/
sudo mv duet3d-unstable.list /etc/apt/sources.list.d/duet3d-unstable.list
sudo chown root:root /etc/apt/trusted.gpg.d/duet3d.gpg
sudo chown root:root /etc/apt/sources.list.d/duet3d-unstable.list
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install with --reinstall duetsoftwareframework

Tested with:
apt list 2>/dev/null | grep duet | awk -F'[/ ]' '{print $1" "$3 }'

Output:
duetcontrolserver 2.0.0
duetruntime 2.0.0
duetsd 1.0.6
duetsoftwareframework 2.0.0
duettools 2.0.0
duetwebcontrol 2.1.3
duetwebserver 2.0.0

Thanks to everyone in this forum, solved it through you.