Posted by If you have “ERROR: Not found source for system packages (release 6.0.3) in APT software sources configuration” during plesk cli installation:
Verify repository in /etc/apt/sources.list and update if necessary
la -la /apt/sources.list.ai_back
if file sources.list.ai_back exist
cp /etc/apt/sources.list /etc/apt/sources.list.ai_back
apt-get update
and re-run the installation